Q&A: What are pathogens, and what have they done to and for us?

Abstract

Microbes are found on us, within us and around us. They inhabit virtually every environment on the planet and the bacteria carried by an average human, mostly in their gut, outnumber human cells. The vast majority of microbes are harmless to us, and many play essential roles in plant, animal and human health. Others, however, are either obligate or facultative pathogens exerting a spectrum of deleterious effects on their hosts. Infectious diseases have historically represented the most common cause of death in humans until recently, exceeding by far the toll taken by wars or famines. From the dawn of humanity and throughout history, infectious diseases have shaped human evolution, demography, migrations and history.

Fig. 1.

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 [–33]; P. falciparum malaria [34, 35]; hepatitis B [36]; measles [37]; plague [38, 39]