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Historical and contemporary views on cholera
transmission: are we repeating past discussions? Can
lessons learned from cholera be applied to COVID-19?
PETER KJÆR MACKIE JENSEN,
1
STEPHEN LAWRENCE GRANT,
1
MADS LINNET PERNER,
1
ZENAT ZEBIN HOSSAIN,
1,2
JANNATUL FERDOUS,
1,2
REBECA SULTANA,
1
SARA ALMEIDA,
1
MATTHEW PHELPS
3
and ANOWARA BEGUM
2
1
1
Center for Disaster Research, Global Health Section, Department of Public Health, University of
Copenhagen, Copenhagen, Denmark;
2
Department of Microbiology, University of Dhaka, Dhaka,
Bangladesh; and
3
The Danish Heart Foundation, Copenhagen, Denmark
Jensen PKM, Grant SL, Perner ML, Hossain ZZ, Ferdous J, Sultana R, Almeida S, Phelps M, Begum A.
Historical and contemporary views on cholera transmission: are we repeating past discussions? Can lessons
learned from cholera be applied to COVID-19?. APMIS. 2020.
Cholera, a devastating diarrheal disease that caused several global pandemics in the last centuries, may share some sim-
ilarities with the new COVID-19. Cholera has affected many populations in history and still remains a significant bur-
den in developing countries. The main transmission route was thought to be predominantly through contaminated
drinking water. However, revisiting the historical data collected during the Copenhagen 1853 cholera outbreak allowed
us to re-evaluate the role of drinking-water transmission in a city-wide outbreak and reconsider some critical transmis-
sion routes, which have been neglected since the time of John Snow. Recent empirical and cohort data from Bangla-
desh also strengthened the dynamic potentiality of other transmission routes (food, fomite, fish, flies) for transmitting
cholera. Analyzing this particular nature of the cholera disease transmission, this paper will describe how the pattern of
transmission routes are similar to COVID-19 and how the method of revisiting old data can be used for further explo-
ration of new and known diseases.
Key words: Cholera transmission; Bangladesh; COVID-19; pandemic; drinking water; F-diagram; Fecal–oral
transmission.
Peter Kjær Mackie Jensen, Center for Disaster Research, Global Health Section, Department of Public Health, Univer-
sity of Copenhagen, Øster Farimagsgade 5a, building 22, 1353 Copenhagen K, Denmark. e-mail: mackie@sund.ku.dk
Cholera, a diarrheal disease caused by bacteria, has
shaped human history for centuries, while COVID-
19 is a newly emergent respiratory disease caused by
the SARS-CoV-2 virus. Despite their apparent dif-
ferences, there are some surprising similarities
between the two diseases. For example, both appear
to spread most easily under crowded conditions
where close, prolonged, personal contact is frequent
and social distancing is not possible or followed.
Due to the recent emergence of SARS-CoV-2,
researchers often find themselves lacking critical
data about the disease, and it falls upon the research
community to explore how and where historical
data can be used to fill these research gaps. To this
end, cholera has taught us the value of looking
backwards to guide the path forward, and historical
data have allowed contemporary researchers to
address long-simmering uncertainties regarding cho-
lera transmission. Therefore, this paper will try to
provide insight into some possibly overlooked
routes that may influence primary cholera transmis-
sion by using both historical and new data, and to
further examine the similarities of these findings to
developing COVID-19 transmission knowledge.
FROM THE SEVEN PANDEMICS TO TODAY
The fear of cholera has, for several centuries, been
one of the drivers behind modern city planning, the
Received 10 September 2020. Accepted 13 November 2020
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APMIS ©2020 APMIS. Published by John Wiley & Sons Ltd.
DOI 10.1111/apm.13102
A P M 13102
Dispatch: 1.12.20 CE: Menaga A
Journal Code Manuscript No.
No. of pages: 10 PE: Nagappan A.R.
demand for adequate living spaces, and better
hygiene, clean drinking water, and sanitation. Cur-
rently, an estimated 1.3 billion people are at risk of
cholera globally, with 1.3 to 4 million cholera cases
occurring annually, which result in approximately
21 000–143 000 fatalities per year (1). Exact figures
are difficult to estimate as many cases go unre-
ported due to political and/or economic reasons
(such as fear of repercussions on trade and tourism)
or simply to the lack of diagnostic capacity in
remote and less economically fortunate areas (2).
Cholera is a severe diarrheal disease caused by the
O1 and O139 strains of the bacterium Vibrio cholerae
and is endemic to the brackish waters in the estuaries
of the Ganga and Brahmaputra rivers in the Bay of
Bengal (3–5). While the first written records of the
disease in South Asia date back to the Hindi Vedas
from 500 BC, the history of cholera suggests the rest
of the world did not know it before 1817. However,
1817 is the year where the first of at least seven differ-
ent O1 pandemics spread from the Bay of Bengal (6).
The current seventh, and still ongoing, pandemic
started in 1961, although not in the Bay of Bengal,
but in Sulawesi, Indonesia. In 1992, a new strain,
O139, emerged in India. After it caused a significant
epidemic in a population that was mostly immune to
the O1 –but not the O139 –serotype, it now resides
in Asia and, for unknown reasons, has not spread to
the rest of the world (7).
Despite its legacy as a lethal disease, with a 25–
50% mortality rate in untreated cases, cholera is
not always a catastrophic public health threat
where it is endemic (8), as proper medical attention
can lower the mortality rate to <1%. The primary
and non-costly treatment of cholera consists of
replacing lost fluids using oral and intravenous-ad-
ministered fluid containing electrolytes (up to 1 liter
per hour). The rice-water stools excreted by symp-
tomatic individuals are highly contagious, and a
person can excrete as many as 2 910
12
V. cholerae
bacteria in a day (9). Volunteer trials have shown
that the infectious dose for severe diarrhea is 10
8
–
10
11
bacteria in healthy persons. The infectious
dose drops to 10
4
–10
8
when a bicarbonate buffer is
used to neutralize the stomach acid shortly before
inoculation, and foods such as rice, fish, custard,
and skimmed milk may act as an acid buffer (10).
However, as many as 50% of people infected with
V. cholerae will never experience severe diarrhea
and (depending on immunity, inoculum dose, etc.)
will only have mild symptoms or be asymptomatic,
although they will still be able to carry and excrete
the bacteria in smaller concentrations (10). As a
consequence of the high bacterial load and volume
of rice-water stool excreted, cholera disproportion-
ately affects people of low socioeconomic status
who live in overcrowded, unhygienic conditions
with insufficient water and sanitation facilities (8).
For this reason, the disease is considered an indica-
tor of economic inequity and a lack of social devel-
opment.
THE IMPACT OF JOHN SNOW
One name that is closely tied to our understanding
of cholera transmission is Dr. John Snow who, in
1854, during the third cholera pandemic in London,
removed the handle of the Broad Street water
pump and thus stopped the outbreak, although the
outbreak had peaked and was already receding
when the handle was removed. In a separate study,
Dr. Snow further showed mortality differences in
households from the same street, but connected to
different waterworks. The two waterworks in opera-
tion were both receiving water from the Thames
river, but had different qualities. The Lamberth
water company inlet was upstream of London’s
sewage outlet, and their customers had a mortality
rate of 37/10 000, while the Southwark and Vaux-
hall Company harvested the polluted water down-
stream with a mortality rate of 315/10 000 (11).
These findings linked water quality to mortality
and, essentially, ended the contemporary arguments
regarding whether contagions or miasmas were the
causes of disease (12). The legend of the pump han-
dle has had an effect on how Dr. Snow’s findings
are remembered, with the linkage between cholera
and contaminated drinking water carved in stone
ever since. His other findings concerning cholera
being linked through person-to-person and food
transmissions appear to have receded into the back-
ground. Even today, short-cycle transmission
(through personal household contact and hygiene)
is under-researched in comparison with long-cycle
transmission (e.g., drinking water) (13).
Cholera can be grouped into two general and
interconnected transmission routes: the aquatic
reservoir to host route (V. cholerae that has sur-
vived and replicated itself outside of a human host
in the aquatic environment and subsequently trans-
mitted to a human host –often referred to as pri-
mary transmission), and the fecal–oral route
(transmission from one human host to another –
often referred to as secondary transmission) (14).
Cholera outbreaks in non-endemic areas such as in
London and Copenhagen in the 1850s are consid-
ered to involve only secondary transmission (with
the potential exclusion of the index case), whereas
cholera outbreaks in endemic areas such as in Ban-
gladesh involve both primary and secondary trans-
mission (8). Despite being researched for decades,
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the importance of the main drivers behind V. c-
holerae primary transmission in Bangladesh remains
disputed, and if potential transmission routes are
not fully explored and understood, the control of
the disease can never be achieved.
NEW HISTORICAL INSIGHT INTO
TRANSMISSION AND THEIR POSSIBLE
IMPLICATIONS
The notion that cholera is closely linked to drinking
water might have been different if John Snow had
lived in Copenhagen in the summer of 1853, where
an outbreak killed more than 4663 people (3.4% of
the city’s population) (15). While no study has
specifically investigated its transmission, most histo-
rians have assumed the outbreak to be drinking
water-driven based on Snow’s Broad Street pump
findings. The Copenhagen outbreak was unique in
an epidemiological sense as the city’s population
had never previously been exposed to V. cholerae.
In the middle of the nineteenth century, Denmark
still had quarantine legislation that had been devel-
oped during the time of the plague. It specified
rules for the quarantine of ships arriving from ports
infected by certain epidemic diseases, cholera
among them. However, in 1852 the quarantine rules
for cholera were lifted due to a change in medical
thought, whereby most authorities decided the dis-
ease was miasmatic and, as such, the quarantine
rules no longer made sense (16).
At the time of the outbreak, Copenhagen had a
population of approximately 130,000 people and a
considerable number of cows, pigs, horses, and
other livestock confined to small, filthy spaces
behind the city’s ramparts. It was not uncommon
to see cows permanently living on the first floors of
city houses, as they were kept to eat the waste
products from beer production. There was no sew-
age system, and all solid and liquid wastes were
temporarily stored in cellars or went directly to the
open gutters flowing through the city and emptying
in the harbor, part of it into the canal in front of
the castle and fish market (16).
Copenhagen had several independent water sup-
plies, all constructed from hollow oak tree pipes
connected by lead joints. In these, the water was
neither treated nor filtered and was free-flowing (by
gravity in non-pressurized systems) from the sur-
rounding lakes to the city. However, one water sup-
ply harvested water from a lake 7 km away from a
height of 30 meters above sea level, which fed the
only water fountain in Copenhagen with pressur-
ized water (17). Leaky lead joints connecting the
pipes allowed wastewater from the gutters in the
streets above to enter the pipes and, hence, the
drinking water. This was revealed in the newspa-
pers where people complained about the water
quality as being filthy, with eels, fish, and other
creatures found in it (18).
In 2018, a project at the University of Copenhagen
modeled the cholera outbreak and paired it with a
GIS overlay of the different water-distribution com-
panies (see Fig. 1) (19). If the pipes were the conveyer
of the disease, cholera would be expected to have
moved with the water flow. The lack of this suggests a
short-cycle transmission within the secondary trans-
mission route in Copenhagen. While the drivers of
this transmission remain unknown, congested hous-
ing, lack of hygiene, and possible food contamination
are qualified guesses. Unfortunately, after 167 years,
it is not possible to investigate the importance of the
different transmission routes further. However, in
Bangladesh, the physical environment, in terms of
crowding and low hygiene housing, is similar to what
could be found in 1853 Copenhagen and London in
1854.
INVESTIGATING TRANSMISSION ROUTES
IN BANGLADESH USING HISTORICAL
OUTBREAKS
The conditions (water temperature, pH, salinity, and
plankton blooms) within the estuaries and in the Bay
of Bengal favor V. cholerae survival and growth at or
around the same periods as seasonal peaks are
observed in the annual 100 000 cholera cases in parts
of Bangladesh (3, 21–23). Therefore, researchers have
investigated whether the contamination of drinking
water from an aquatic reservoir was the main route of
cholera transmission in Bangladesh (3, 22, 24), either
by drinking water drawn directly from the rivers dur-
ing the dry season, or by drawing from water wells
that were flooded in the monsoon season. Conversely,
King et al. (25) used mathematical modeling to argue
that free-living V. cholerae in the aquatic reservoir
was responsible for relatively few numbers of cholera
cases. Their study suggested that previously underes-
timated numbers of mild or asymptomatic cases hold
the key to interpreting the patterns of disease in Ban-
gladesh.
FROM AQUATIC RESERVOIR TO HOST IN
BANGLADESH (PRIMARY TRANSMISSION)
Aquatic reservoir
In Bangladesh, V. cholerae can be extracted from
aquatic reservoirs through water and fish (3, 26). In
other countries, water plants and water birds have
©2020 APMIS. Published by John Wiley & Sons Ltd 3
VIEWS ON CHOLERA TRANSMISSION
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also been suggested as potential extraction points
(8, 24, 27).
To become infected with cholera from an aquatic
reservoir, a host needs to ingest V. cholerae via
water (such as through consuming contaminated
drinking water, or bathing/swimming in a contami-
nated river and ingesting some of the water), food,
contact with fomites (such as kitchen utensils) or
direct oral contact with his/her hands (e.g.,, a per-
son may eat with contaminated hands). Thus, we
can ask, what is the link between the points where
V. cholerae can be extracted from an aquatic reser-
voir and the points where it can be ingested by the
host? (Fig. 2).
Fig. 1.
4The different water companies pipes (different colored) followed the gradient (not shown) of the landscape and flo-
wed from the lakes towards the harbor and the castle (from upper left corner diagonally to the lower right), while cholera
crossed the pipe networks and moved horizontally (upper right to lower left) in the city (20)
2.
LOW RESOLUTION COLOR FIG
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Water extracted from the aquatic reservoir for
drinking water
Given the distinct seasonal patterns of cholera cases
in Bangladesh, researchers have investigated
whether there are seasonal introductions of V. c-
holerae into the drinking water directly from the
aquatic reservoir (3, 21, 22, 29, 30). Every year, low
river flow during the dry season leads to an intru-
sion of saline water from the Bay of Bengal into
the coastal regions, creating river environments
conducive to V. cholerae survival and growth.
Some studies have suggested these conditions spark
the coastal dry season outbreaks once people ingest
contaminated water originating from the aquatic
reservoirs (3, 21). As the dry season progresses, sal-
ine water intrudes further inland and, with the aid
of copepods, V. cholerae migrates to the inland
areas. During the monsoon season, floodwaters
carry V. cholerae from the rivers to the inundated
inland regions where it then proliferates in water-
logged areas. Research has suggested that the flood
waters contaminate water sources, including ponds
and wells used for drinking water, thereby driving
the post-monsoon outbreak in the inland areas (3).
The rivers could be a direct transmission route
from the aquatic reservoir to the host, but only if
high salinity water was found to be consumed by
the local population (8). Unsurprisingly, however,
Bangladeshis tend to choose drinking water that
has an authentically pleasing taste (23). Therefore,
would people in Bangladesh drink water directly
from a high salinity aquatic reservoir? To answer
this question, Grant et al. (31) conducted a simple
taste experiment investigating whether a local popu-
lation from the coastal region of Bangladesh had
the tolerance to drink water with a salinity similar
to that which is found in the rivers during the low
flow season. It was further investigated whether
they would continue to drink from a water source
after it had been flooded and contaminated with
brackish river water during the monsoon season.
The results for both scenarios were negative, indi-
cating that there might be other, or parallel, routes
of transmission.
Cholera transmission via the consumption of fish
Studies outside Bangladesh have found associations
between cholera and the consumption of under-
cooked shellfish (8, 24, 32). However, in Bangla-
desh, most shellfish are exported, expensive for
poor households, and harvested during periods that
do not coincide with the cholera seasons (33).
Therefore, while it is plausible that some cases
could occur through this route, it is unlikely to be
the driving force behind the large seasonal out-
breaks observed in the Bangladeshi context.
To investigate the possible transmission via fish,
a study in 2018 found a high prevalence of V. c-
holerae in the gills, recta, intestines, and scales of
the Hilsa fish –a commonly caught and consumed
fish in Bangladesh that lives in both freshwater and
seawater (26). In 1951, Pandit and Hora had
Fig. 2. Cholera’s possible primary transmission routes in Bangladesh, from the aquatic reservoir to the host (28).
©2020 APMIS. Published by John Wiley & Sons Ltd 5
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mentioned Hilsa fish as a possible link, but their
results were not investigated further (34). Hilsa fish
migrate up the rivers from the Bay of Bengal twice
a year, from July to October and January to
March, which corresponds to the two seasonal
peaks in cholera cases (29). This suggests that Hilsa
fish may potentially serve as a vehicle of transmis-
sion for V. cholerae (26). Furthermore, during these
periods, the availability of Hilsa fish in the local
markets increases, resulting in lower prices. In con-
trast, during other times of the year, the prices are
generally too high for poor households (26). Fish in
Bangladesh is cooked or fried before eating and,
therefore, cannot serve as a direct transmission
route. However, they could represent a possible
missing transmission link: unhygienic conditions
within the kitchen environment and consequent
kitchen transmission.
Unhygienic conditions within the kitchen environment
as a potentially overlooked transmission link
In Bangladesh, studies have shown that the kitchen
environment could be contaminated through the
cleaning and gutting of contaminated fish (26), or
through the use of contaminated river or canal
water for domestic purposes (34). Likewise, using
domestic water contaminated with V. cholerae on
prepared food and vegetables that are eaten
uncooked has also been linked to cholera transmis-
sion (35).
In Bangladesh, fish is generally purchased whole
in the local markets and then taken home to be
cleaned and gutted. This is often done on the floor
of kitchen areas. Utensils, including a large station-
ary cutting knife (called ’boti’), are typically used
for cutting uncooked fish and other food in the
kitchen (26). In a hygiene study by Hussain et al.
(36), a considerable amount of E. coli contamina-
tion was found on botis and washed food plates.
Other studies have suggested that the unhygienic
handling of fish may be another factor leading to
V. cholerae transmission (8, 24). Scheelbeek et al.
(37) suggested that in conditions where kitchen
hygiene is limited, there is a higher risk of cholera
transmission from cleaning fish than from consum-
ing them.
Once V. cholerae is present in the kitchen envi-
ronment, unhygienic conditions may allow for the
cross-contamination/recontamination of food and
water through multiple routes. For example, one
study found that V. cholerae survived on fomites
typically used in kitchen environments for 1–4hin
a culturable state and up to 7 days in a viable but
non-culturable state (i.e., still infectious but unable
to reproduce) (38). A limited amount of available
water in the kitchen environment may restrict peo-
ple’s ability to engage in safe hygiene practices such
as hand washing or thoroughly cleaning kitchen
tools/utensils (39), thereby increasing the risk for
cross-contamination of V. cholerae from fomites to
food and/or water and, ultimately, transmission to
a new host.
THE FECAL–ORAL ROUTE (SECONDARY
TRANSMISSION)
To further understand fecal–oral transmission, in
1958 Wagner and Lanoix (40) developed the F-dia-
gram, illustrating how a pathogen may be transmit-
ted from feces to a new host through fingers, flies,
fields, fluids, and food. Within the F-diagram, each
element represents the transmission route of the
pathogen. In the finger route, pathogens are trans-
mitted from the original host’s feces to food or
directly to a new host via hand contact. Systematic
reviews have supported these results by finding that
hand washing with soap has a protective effect
against diarrhea diseases (41). However, studies in
Bangladesh have shown that, among cholera
patients, there is a low frequency of hand washing
with soap (7%) after toilet visits (42). Flies can also
act as vectors that carry bacteria by first landing on
feces and then on food. A study conducted in a
slum area of Dhaka confirmed the transmission of
V. cholerae by flies in a real kitchen environment
and quantified Escherichia coli transmission during
food preparation, which showed a magnitude of
600 E. coli deposited in 50% of the landings (43,
44). In the field route, the transmission of patho-
gens may occur when untreated feces are used as
fertilizer or wastewater is used for crop irrigation
(45). The fluid route can be divided into drinking
and domestic water such as water used for cooking,
personal hygiene, and household cleaning. The
drinking-water route is relatively straightforward,
and occurs when a drinking-water source is con-
taminated by feces containing V. cholerae and is
then consumed by a new host. However, one limita-
tion of the F-diagram is that it does not account
for possible recontamination (contamination occur-
ring within the household) of drinking water that
has been previously treated or collected from a
’clean and safe’ source when it is drunk from a
dirty glass or stored under unhygienic conditions
(46, 36).
Evidence has also been found that V. cholerae
can survive for long periods, and even replicate, on
food under the right conditions (7). In Bangladesh,
cholera transmission has been associated with
uncooked vegetables as well as prepared foods such
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as cooked rice and chicken, when these food items
have come into contact with contaminated fingers,
flies, or fluids (47).
The Copenhagen transmission was secondary,
while the outbreaks in Bangladesh have a primary
source. However, we can use the analysis of histori-
cal, secondary transmission in the analysis of pri-
mary transmission, especially when we understand
that other, non-drinking, water-borne transmissions
might be important. This idea leads us to Fig. 3,
which illustrates where primary and secondary
transmission can be interrupted.
Fighting cholera is about disrupting V. Cholerae
transmission routes, and Fig. 3 provides an over-
view of where and by what means such transmis-
sion routes can be broken. As illustrated, a
successful intervention likely involves different
approaches, but first, the routes need to be qualified
in terms of their relative importance in bacterial
transmission to evaluate the possible effect of the
intervention. While these lines of disruption are
specific to Bangladesh in terms of water use and
habits, they are based on a historical approach
using the data from Copenhagen in 1853. To defini-
tively distinguish and quantify the specific transmis-
sion route in which a host has become infected
with V. cholerae, a thorough investigation using
microbiological testing and contact tracing is
required. However, in lower-income settings such
Bangladesh, this is rarely possible.
John Snow showed that water quality is impor-
tant. However, by looking at infectious doses, it is
also clear that only heavily contaminated water is
able to make healthy people ill. In contrast, only
0.1% of the V. cholerae infectious dose in drinking
water is needed if it is ingested via food. Therefore,
it is essential to revisit the original ideas of John
Snow, and not simply focus on water quality. This
means enlarging the focus to include the house, and
especially kitchen hygiene, even though improve-
ments in housing standards and kitchen upgrades
are more expensive than simply chlorinating the
water supply.
HISTORICAL SIMILARITIES TO COVID-19
Despite the dissimilarities in the causes of COVID-
19 (a zoonotic virus) and cholera (bacteria), there
are several similarities that can be observed. First,
both are seen as society-disrupting, Asian-origin
diseases spreading out to the rest of the world. Sec-
ond, neither disease is entirely new in the sense that
human coronaviruses, and the original O1 cholera
type, are known diseases with considerable immu-
nity among previously exposed populations. How-
ever, after their initial appearances, the continuous
evolution of the SARS coronavirus and V. cholerae
created new outbreaks, and both SARS-CoV-2 and
O-139 have quickly become new threats to public
health. Moreover, numerous asymptomatic carriers
make the fight against both diseases more challeng-
ing.
Is COVID-19 is a poverty-driven dis-
ease? Cholera is known to be a disease of poverty,
and current data have indicated that COVID-19,
despite being able to cause worldwide infections
across socioeconomic boundaries, tends to have a
higher mortality rate in lower-income populations
(48, 49). As these population groups have a ten-
dency to live in overcrowded conditions where
social distancing and high hygiene levels are impos-
sible to maintain. Therefore, compliance with typi-
cal public health advice on how to reduce the
spread of the virus is likely to be low among these
populations. However, infrastructural risk factors,
Fig. 3. Cholera’s primary (reservoir to host) and secondary (feces to new host) transmission routes in Bangladesh, with
redlines as indicators for possible interventions (28).
COLOR
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that is, hygiene and housing, are not the only risks.
Access to health care and the presence of under-
treated, non-communicable diseases such as dia-
betes, which has a higher prevalence among lower
socioeconomic groups (51), are other factors to be
taken into consideration. Both COVID-19 and cho-
lera have had and will have the potential to change
future living conditions, the way we interact, and
especially how we plan and build future standards
for living/work space and hygiene.
As to the possible fecal–oral transmission of
SARS-CoV-2, it remains unclear. While live virus
has been detected in many infected patients’ stool
samples (52, 53), it is uncertain if it remains infec-
tious. Present knowledge regarding the spread of
COVID-19 via fields and fluids is also limited, and
the contribution of fomites and flies is currently
being investigated (54–58). The presence of SARS-
CoV-2 in wastewater has also been widely discussed
(59). However, no infectious virus has been detected
in either untreated or treated sewage, or in drink-
ing-water supplies.
The importance of airborne transmission of
SARS-CoV-2 and, specifically, the importance of
transmission via droplets vs. aerosols (60) illustrate
a key challenge for both diseases. That is, although
we can recognize contamination routes, we are not
fully aware of their relative importance, that is,
which routes are the most potent in terms of the
flow of pathogens and the actual infective dose,
and which ones exist but are insignificant from a
broader public health perspective (61). Perhaps the
clue to answer most of our current questions
regarding cholera and COVID-19 does not lie
entirely within new investigations, but in looking
back and re-analyzing the vast amount of data that
has been collected in the past.
Hopefully, this paper will inspire researchers to
revisit the old epidemics and learn from them, as
now, when time is of the essence, is the moment to
stand on the shoulders of our forebearers and look
not only at their discoveries, but also at their com-
piled work, because ‘people will not look forward
to posterity who never look backward to their
ancestors’. –Edmund Burke.
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