It is difficult to rank the common infectious diseases in humans. Some organisms have a very high rate of infection, but produce a relatively low rate of clinical disease and death. Other organisms have relatively low levels of infection, but have a very high virulence, resulting in many deaths.
What follows is a listing of the most common infections that occur in humans, beginning with the organisms that can be found within the majority of humans (e.g., greater than 3.5 billion), and ending with infections involving more than one million individuals. Some infections that deserve to be included here (e.g., yellow fever) are omitted for lack of finding a trusted historical data source.
Infections occurring in the majority of humans (i.e., 3.5 to 7 billion cases).
- Demodex is a tiny mite that lives in facial skin. Demodex mites can be found in the majority of humans.
- The BK polyomavirus rarely causes disease in infected patients, and the majority of humans carry the latent virus.
- The JC polyomavirus persistently infects the majority of humans, but it is not associated with disease in otherwise healthy individuals.
Infections occurring in 1 to 3.5 billion humans.
- About two billion people (of the world's 7 billion population) have been infected with Mycobacterium tuberculosis.
- About one third of human population has been infected (i.e., about 2.3 billion people) by the only species that produces human toxoplasmosis: Toxoplasma gondii.
- Ascaris lumbricoides, the cause of ascariasis, infects about 1.5 billion people worldwide, making it the most common helminth (worm) infection of humans (1).
Infections involving 500 million to 1 billion humans.
- Various estimates would suggest that worldwide, more than half a billion people are infected with one or another subtypes of Chlamydia trachomatis. This would include the various Chlamydia organisms and serotypes that account for trachoma and chlamydial urethritis. According to the World Health Organization, there are about 37 million blind persons, worldwide. Trachoma, caused by Chlamydia trachomatis, is the number one infectious cause of blindness and accounts for about 4% of these cases. The second most common infectious cause of blindness worlwide is Onchocerca volvulus, accounting for about 1% of cases (2).
- About 200 million people are infected by schistosomes (i.e., have some form of schistosomiasis).
- Hookworms infect about 600 million people worldwide. Two species are responsible for nearly all cases of hookworm disease in humans: Ancylostoma duodenale and Necator americanus.
- Scabies is an exceedingly common, global disease, with about 300 million new cases occurring annually.
Infections involving 100 million to 500 million humans.
- Hepatitis B infects more than 200 million people, worldwide, causing two million deaths each year.
- Bubonic plague is credited with killing one third of the population of Europe in the mid-1300s. Altogether, bubonic plague is estimated to have caused about 200 million deaths. In modern times, plaque is rare, but not extinct. Each year, several thousand cases of plague occur worldwide, resulting in several hundred deaths. Virtually all of the contemporary cases occur in Africa.
- Genus Plasmodium is responsible for human and animal malaria. About 300 - 500 million people are infected with malaria, worldwide, causing 2 million deaths each year (3) (4).
- About 150 million people are infected by the filarial nematodes (genera Brugia, Loa, Onchocerca, Mansonella, and Wuchereria) (5). Wuchereria bancroft and Brugia malayi, together, infect about 120 million individuals (5). Most cases occur in Africa and Asia.
- Smallpox is reputed to have killed about 300 million people in the twentieth century, prior to the widespread availability of an effective vaccine. Smallpox, now extinct, has been referred to as the greatest killer in human history.
- Worldwide, about 100 million cases of acute diarrhea are caused by rotavirus. In 2004, rotavirus infections accounted for about a half million deaths in young children, from severe diarrhea (6).
Infections involving 10 million to 100 million humans.
- The 1917-1918 influenza pandemic caused somewhere between 50 million and 100 million deaths. Seasonal influenza kills between a quarter million and a half million people worldwide, each year. In the U.S., seasonal influenza accounts for about 40,000 deaths annually.
- It is estimated that about 50 million people are infected by Entamoeba histolytica, with about 70,000 deaths per year, worldwide.
- Paragonimus westermani, along with dozens of less frequent species within Genus Paragonimus, causes the condition known as paragonomiasis. About 22 million people are infected worldwide, with most cases occurring in Southeast Asia, Africa, and South America.
- More than 50 million dengue virus infections occur each year, causing about 25,000 deaths worldwide. Most infections are asymptomatic or cause only mild disease. A minority of cases are severe.
- Between 1918 and 1922, epidemic, louse-borne, typhus (Rickettsia prowazekii) infected 30 million people, in Eastern Europe and Russia, accounting for about three million deaths (7).
- The most common sexually transmitted disease is trichomoniasis (Class Metamonada), with about 8 million new cases each year in North America (8). The second most common sexually transmitted disease is chlamydia (Class Chlamydiae), with about 4 million new cases each year in North American (8). Approximately 1.5 million new cases of gonorrhea occur annually in North America, where gonorrhea is the third most common sexually transmitted disease (8). According to the U.S. Centers for Disease Control and Prevention, there were about 46,000 new cases of syphilis and 48,000 new cases of HIV reported in the U.S., in 2011 (9).
- Leprosy, also known as Hansen disease, is caused by Mycobacterium leprae and Mycobacterium lepromatosis. From the 1960s to the 1980s, the number of leprosy cases worldwide was a steady 10-12 million (10) The introduction of effective multidrug protocols has resulted in many cured cases and in lowered infection rates. Consequently, the number of cases of leprosy has dropped to about 5.5 million worldwide in the 1990s (10). In 2005, there were about 300,000 new cases reported, worldwide (11).
- Leishmania species cause leishmaniasis, a disease that infects about 12 million people worldwide. Each year, about 60,000 people die from the visceral form of the disease.
- Trypanosoma cruzi is the cause of Chagas disease, also known as American trypanosomiasis. Chagas disease affects about eight million people (12). Trypanosoma brucei is the cause of African trypanosomiasis (sleeping sickness). The reported numbers of cases are may be somewhat unreliable, but it has been estimated that infection with Trypanosoma brucei accounts for about 50,000 deaths each year.
- Fasciolopsiasis is caused by Fasciolopsis buski, a large (us to 7.5 cm. length) fluke that lives in the intestines of the primary host, pigs and humans (See Glossary item, Primary host). The number of humans infected is about 10 million.
- Clonorchis sinensis, the Chinese liver fluke (also known as the Oriental liver fluke) infects about 30 million people.
- In the year 2000, measles caused approximately nearly 40 million illnesses and about 750,000 deaths worldwide (13).
What kinds of organisms cause infections? All types. It seems to be a condition of terrestrial life that organisms live within one another. Humans can be infected by any of the classical kingdoms of living organisms: Bacteria, Fungi, Animalia, Plantae, and members of the kingdom formerly known as Protoctista, containing the protozoan parasites. Humans can also be infected by non-living organisms (i.e., virus, prion). In my book "Taxonomic guide to infectious diseases: understanding the biologic classes of pathogenic organisms," most of the known infections of humans are described, with their culpable organisms assigned to their proper phylogenetic classes (14).
 Crompton DW. How much human helminthiasis is there in the world? J Parasitol 85:397-403, 1999.
 Resnikoff S, Pascolini D, Etyaale D, Kocur I, Pararajasegaram R, Pokharel GP, et al. Global data on visual impairment in the year 2002. Bulletin of the World Health Organization 2004;82:844-851, 2004.
 The state of world health. Chapter 1 in World Health Report 1996. World Health Organization. Available from: http://www.who.int/whr/1996/en/index.html 1996.
 Lemon SM, Sparling PF, Hamburg MA, Relman DA, Choffnes ER, Mack A. Vector-Borne Diseases: Understanding the Environmental, Human Health, and Ecological Connections, Workshop Summary. Institute of Medicine (US) Forum on Microbial Threats. Washington (DC): National Academies Press (US). 2008.
 Foster J, Ganatra M, Kamal I, Ware J, Makarova K, Ivanova N, et al. The Wolbachia genome of Brugia malayi: endosymbiont evolution within a human pathogenic nematode. PLoS Biol 3:e121, 2005.
 Weekly epidemiological record. World Health Organization. 32:285-296, 2007.
 Cowan G. Rickettsial diseases: the typhus group of fevers: a review. Postgrad Med J 76:269-272, 2000.
 Global prevalence and incidence of selected curable sexually transmitted infections: overview and estimates. World Health Organization. Geneva. 2001.
 Sexually transmitted diseases. U.S. Centers for Disease Control and Prevention. Available from http://www.cdc.gov/std/syphilis/stdfact-syphilis.htm, viewed October 24, 2013.
 Noordeen SK, Lopez Bravo L, Sundaresan TK. Estimated number of leprosy cases in the world. Bull World Health Organ 70:7-10, 1992.
 World Health Organization. Global leprosy situation. Weekly Epidemiological Record 81:309-16, 2006.
 Rassi A Jr, Rassi A, Marin-Neto JA. Chagas disease. Lancet 375:1388-1402, 2010.
 Stein CE, Birmingham M, Kurian M, Duclos P, Strebel P. The global burden of measles in the year 2000: a model that uses country-specific indicators. J Infect Dis 187:S8, 2003.
 Berman JJ. Taxonomic Guide to Infectious Diseases: Understanding the Biologic Classes of Pathogenic Organisms. Academic Press, Waltham, 2012.
- Jules Berman (copyrighted material)
key words: infectious diseases, incidence of disease, infection, jules j berman