An updated literature survey identified 1,407 recog-
nized species of human pathogen, 58% of which are
zoonotic. Of the total, 177 are regarded as emerging or
reemerging. Zoonotic pathogens are twice as likely to be in
this category as are nonzoonotic pathogens. Emerging and
reemerging pathogens are not strongly associated with
particular types of nonhuman hosts, but they are most like-
ly to have the broadest host ranges. Emerging and
reemerging zoonoses are associated with a wide range
of drivers, but changes in land use and agriculture and
demographic and societal changes are most commonly
cited. However, although zoonotic pathogens do represent
the most likely source of emerging and reemerging infec-
tious disease, only a small minority have proved capable of
causing major epidemics in the human population.
than half known to be zoonotic, i.e., able to infect other
host species (1,2). The survey data showed that those
pathogens regarded as emerging and reemerging were
more likely to be zoonotic than those that are not (1,3),
confirming an association between these characteristics
which had long been suspected (4,5), but which could not
be formally demonstrated without denominator data as
well as numerator data.
Here, we revisit these calculations, using updated infor-
mation on the biology and epidemiology of recognized
human pathogens. We pay close attention to possible dif-
ferences between the major pathogen groups—viruses,
bacteria, fungi, protozoa, and helminths. We also examine
in detail the relationship between host range and pathogen
emergence or reemergence, considering both the type and
diversity of nonhuman hosts. We catalog the kinds of prox-
imate factors or drivers that have been linked with
pathogen emergence and reemergence and ask whether
recent, comprehensive literature survey of human
pathogens listed >1,400 different species (1), more
these differ between the major pathogen groups or between
zoonotic and nonzoonotic pathogens.
We focus mainly on pathogen diversity (as numbers of
species) rather than on the effects of disease that they
impose, noting that many diseases, e.g., infant diarrhea,
can be caused by more than one species of pathogen.
However, we comment on the transmissibility of
pathogens once they have been introduced into the human
population because transmissibility is an important deter-
minant of the potential public health problem.
We obtained counts of pathogen species from an updat-
ed version of the previously published database (1). As
before, we defined a human pathogen as “a species infec-
tious to and capable of causing disease in humans under
natural transmission conditions.” We included pathogens
that have only been reported as causing a single case of
human disease and those that only cause disease in
immunocompromised persons. We also included instances
of accidental laboratory infection but excluded infections
resulting from deliberate exposure in the laboratory. We
added recently recognized pathogens listed online by the
Centers for Disease Control and Prevention, the World
Health Organization (WHO), ProMED, and elsewhere
(6–9). We obtained taxonomic classifications online from
the International Committee on Taxonomy of Viruses, the
National Centre for Biotechnology Information, the CAB
International Bioscience database of fungal names, and
from standard texts (10–15).
Pathogen species were categorized as emerging or
reemerging based on previously published reviews of the
literature (1,3), again updated from online sources (6–8). A
species was regarded as emerging or reemerging if any
recognized variant fell into this category (e.g., Escherichia
coli O157, H5N1 influenza A).
We considered the following pathogen groups: viruses
(including prions), bacteria (including rickettsia), fungi
Host Range and Emerging and
Mark E.J. Woolhouse* and Sonya Gowtage-Sequeria*
1842Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 12, December 2005
*Centre for Infectious Diseases, University of Edinburgh,
Edinburgh, United Kingdom
10. International Committee on the Taxonomy of Viruses. Index virum.
[cited 10 May 2005]. Available at http://life.anu.edu.au/viruses/
11. National Center for Biotechnology Information. Taxonomy browser.
[cited 10 May 2005]. Available at http://www.ncbi.nlm.nih.gov/
12. CAB International Bioscience. Index fungorum. [cited 10 May 2005].
Available at http://184.108.40.206/Names/Names.asp
13. Collier L, Balows A, Sussman M, editors. Topley & Wilson’s
Microbiology and Microbial Infection, Volume 4. London: Arnold;
14. Schmidt GD, Roberts LS. Foundations of parasitology, 6th ed.
London: McGraw-Hill; 2000.
15. Mayo MA. A summary of taxonomic recently approved by ICTV.
Arch Virol. 2001;147:1655–63.
16. World Health Organization. Zoonoses: second report of the joint
WHO/FAO expert committee. Geneva: The Organization; 1959.
17. Woolhouse ME, Dye C. Population biology of emerging and re-
emerging pathogens —preface. Philos Trans R Soc Lond B Biol Sci.
18. World Organization for Animal Health. Terrestrial animal health
code—2005. General definitions. [cited 1 Sep 2005]. Available at
19. Hay SI, Guerra CA, Tatem AJ, Noor AM, Snow RW. The global dis-
tribution and population at risk of malaria: past, present and future.
Lancet Infect Dis. 2004;4:327–36.
20. Corbett EL, Watt CJ, Walker N, Maher D, Williams BG, Raviglione
MC et al. The growing burden of tuberculosis: global trends and
interactions with the HIV
21. Burke DS. Evolvability of emerging viruses. In: Pathology of emerg-
ing infections 2, Nelson AM, Horsburgh CR, editors. Washington:
American Society for Microbiology; 1998. p. 1–12.
22. Woolhouse MEJ, Haydon DT, Antia R. Emerging pathogens: the epi-
demiology and evolution of species jumps. Trends Ecol Evol.
23. Woolhouse MEJ. Population biology of emerging and re-emerging
pathogens. Trends Microbiol. 2002;10:S3–7.
24. Dobson A, Foufopoulos J. Emerging infectious pathogens of wildlife.
Philos Trans R Soc Lond B Biol Sci. 2001;356:1001–12.
epidemic. Arch Intern Med.
Address for correspondence: M.E.J. Woolhouse, Centre for Infectious
Diseases, University of Edinburgh, Ashworth Laboratories, Kings
Buildings, West Mains Rd, Edinburgh EH9 3JT, UK; fax: 44-131-650-
6564; email: email@example.com
Host Range and Emerging and Reemerging Pathogens
Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 12, December 20051847