How many pathogens

Age of emergence of significant infectious diseases impacting on human populations. The time of emergence for the various major diseases is based on a synthesis of published research. When known with some confidence, point estimates are provided for each disease together with error bars depicting uncertainty in the inferred estimates. Orange error bars depict higher uncertainty compared to red. The black trend line plots an increase in human population size through time x axis in the order of billions of people y axis.

Key events in human history are highlighted and annotated at the top. Main references used were: smallpox [ 27 ]; influenza [ 28 ]; HIV [ 29 ]; tuberculosis [ 30 , 31 , 32 , 33 ]; P. Infectious diseases have killed well over half of all humans who have ever lived on earth. Pathogens, such as childhood diseases, that affect their host prior to reproduction, through death or reduced fertility, have exerted enormous selective pressures. Yet, scans of the genome for signatures of pathogen-driven selection have identified only a few variants with clear effects.

Even for diseases that have affected us for a long time, for example tuberculosis, we know of no obvious protective genetic variant. Given the high selective pressure pathogens must have exerted, it is reasonable to ask where all the resistance genes are. Strongly protective variants may have reached fixation, rendering them undetectable unless the pathogen has a highly heterogeneous distribution range.

Another situation where a resistance gene does not reach fixation arises when the protective variant is deleterious when homozygous, as in sickle cell anaemia. We might also speculate that the evolutionary potential and high genetic diversity of most pathogens limits our ability to detect protective variants in the human genome, particularly so if these were only effective against a subset of lineages within a pathogenic species.

In addition to the few variants protective against specific pathogens, we also know of genomic regions involved in immunity against a wide spectrum of pathogens, such as interleukin genes or the major histocompatibility complex MHC system.

The very high genetic diversity of the MHC is believed to have been shaped by exposure to different pathogen species [ 18 ]. Also, following the recent development of techniques to sequence ancient DNA, it has been suggested that immunity genes such as those encoding toll-like receptors have been acquired following hybridization with archaic humans and are over-represented in the current gene pool of anatomically modern humans relative to genes not involved in immunity [ 19 , 20 ].

Infectious diseases had a massive impact on our history, leading to the rise and fall of civilizations, both through the toll they took on human life but also through economic and societal collapse following epidemics. Thucydides reports in his History of the Peloponnesian War , written in the 5 th century BC, how the plague of Athens devastated the city-state of Athens in ancient Greece during the second year of the Peloponnesian War BC when it was on the cusp of victory against Sparta, ending the golden age of Pericles and Athenian predominance in the ancient world.

The eventual fall of the Roman Empire was also largely down to another epidemic, the Justinian plague in — CE, which precluded the Emperor Justinian recovering lost territories in the western part of the empire [ 21 ].

Infectious disease played an equally important role in past human migrations. Conversely, one of the possible reasons Europeans managed to colonize Africa was that they used quinine, an antimalarial drug derived from the bark of the cinchona tree [ 23 ]. History has been shaped not only by pathogens infecting humans, but also those affecting domestic animals and crops. For example, it has been suggested that the Islamic conquest of the 7 th and 8 th centuries did not extend to Sub-Saharan Africa because the horses and camels of the Islamic armies were dying from trypanosma spread by tsetse flies [ 24 ].

Conversely, pathogens were at other times the drivers of large migration. Around one million Irish people died and another million migrated to the US to escape the famine caused by Phytophthora infestans destroying potato harvests between and [ 25 ].

At least in the developed world, the leading causes of human mortality are no longer infectious diseases but instead age-associated disorders such as cancer, heart disease and diabetes. Numerous countries have undergone an epidemiological transition, starting some years ago in some developed countries and less than 80 years ago for developing countries.

Diseases that once devastated human populations, such as smallpox, are now eradicated. Others, such as the plague or leprosy, are largely under control with the exception of a few hotspots. The current situation is, however, one of new challenges. Globalization and increased mobility, particularly air travel, have facilitated the transmission of diseases not just locally but between continents. The recent outbreak of Zika in the Americas, for example, has been attributed in part to an increase in air travel from infected areas into Brazilian airports, extending both the incidence and geographic range of the virus [ 26 ].

The outbreak of severe acute respiratory syndrome SARS and recurrent Ebola crises in Central Africa highlight the ability of new and existing diseases rapidly to become significant international health threats. In addition, our ability to combat infectious diseases is also challenged by the widespread emergence of pathogen drug resistance.

The global antimicrobial resistance AMR crisis is increasingly limiting our resources to combat disease through antimicrobial therapy. Thus, in spite of the global health narrative supporting a decline in the number of deaths caused by infectious disease, the complexity of our interactions with disease-causing agents are as significant now as through history.

Infectious diseases continue to be a major cause of mortality globally, responsible for between a quarter to a third of all deaths and nearly half of all deaths in people under the age of 45, with most of these in principle avoidable. Microbiology by numbers. Nat Rev Microbiol. Revised estimates for the number of human and bacteria cells in the body. PLoS Biol. Google Scholar. The role of damselflies Odonata: Ztgoptera as paratenic hosts in the transmission of Halipegus eccentricus Digenea: Hemioridae to Anurans.

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Featured content. Free courses. All content. Course content. Session 3. About this free course 12 hours study. Level 1: Introductory. Course rewards. Free statement of participation on completion of these courses. Create your free OpenLearn profile. Course content Course content. Infection and immunity Start this free course now. Free course Infection and immunity. Table 1 Pathogen types and the number that cause infectious diseases in humans, estimated in Pathogen type Number causing human disease Parasites: multicellular Protists: single-celled 57 Fungi: e.

Previous Introduction. Next 3. Take your learning further Making the decision to study can be a big step, which is why you'll want a trusted University. Fungi can be found just about everywhere in the environment, including indoors, outdoors, and on human skin.

They cause infection when they overgrow. Fungi cells contain a nucleus and other components protected by a membrane and a thick cell wall. Their structure can make them harder to kill.

Some new strains of fungal infections are proving to be especially dangerous , such as Candida aurus , and have prompted more research into fungal infections. Parasites are organisms that behave like tiny animals, living in or on a host and feeding from or at the expense of the host. Though parasitic infections are more common in tropical and subtropical regions, they can occur anywhere.

They can be spread several ways, including through contaminated soil, water, food, and blood, as well as through sexual contact and via insect bites. Viruses can cause a number of infections, many of which are contagious. Examples of viral diseases include:. Pathogens have the ability to make us sick, but when healthy, our bodies can defend against pathogens and the illnesses they cause. Treatments are available for many of the illnesses caused by the different types of pathogens.

Bacterial meningitis is a serious disease that can lead to paralysis, stroke, and even death. Read more on how to treat and prevent it. Airborne diseases spread easily and are difficult to prevent. Learn more about the types of airborne diseases and how to protect yourself. The 5-second rule gives you permission to eat something that fell on the floor, as long as it's picked up within five seconds. But is it safe? Boils are caused by bacteria building up in a hair follicle and pushing up to the surface of the skin.

Recurring boils happen for a number of reasons…. Certain E. Learn about other bacteria and parasites like pinworms and how to prevent…. Shigellosis is a bacterial infection that affects the digestive system.