The emergent discipline of evolutionary epidemiology recognizes that pathogens (disease-causing organisms) are biological entities that can evolve quickly to take advantage of ecological or environmental changes that promote their transmission. Virulence, a measure of a microorganism's ability to infect and sicken a host, also evolves, which allows previously mild diseases to become life-threatening ones, or vice versa. Pathogens and virulence evolve because the life cycle of pathogens is typically very short compared to that of their hosts. The majority of bacteria and viruses replicate numerous times daily, enabling natural selection to favor variants that thrive against the relatively stable, more slowly adapting host. In addition, virulence varies not only from pathogen to pathogen, but also from strain to strain. For example, Escherichia coli can cause fatal foodborne illnesses, yet harmless strains of this bacterium inhabit the intestinal tract of humans. Thus, evolutionary epidemiologists seek to explain the factors contributing to the evolution of pathogens and virulence. See also: Bacteria; Disease; Epidemiology; Escherichia; Escherichia coli outbreaks; Food safety and foodborne illness; Infectious disease; Organic evolution; Pathogen; Virulence; Virus

Pathogens spread either by vertical transmission (in which parents pass pathogens to their offspring either during reproduction or parental care) or horizontal transmission (within one host generation among individuals who need not be related). Higher levels of virulence often correlate with greater transmission, but not always: The details of the pathogen and host life cycles and the environments that they share exert a powerful influence. Pathogens will go extinct if they become so virulent that their hosts die before passing on the infection. Consequently, in pathogens that depend on one sick host infecting others directly, virulence often evolves to an intermediate level that balances transmission rates against host mortality. However, the population density of the hosts also plays an important part in this calculation: In densely populated areas, where sick hosts may pass on pathogens more easily, virulence may increase. The evolutionary pressure to moderate virulence may also be lower for airborne pathogens that can drift on the wind and for pathogens such as the malaria parasite, which moves among human hosts through a secondary mosquito host that does not become ill. In addition, the adaptation of pathogens to new hosts can complicate the interdependency of virulence and transmission. Movements of wildlife diseases to humans, including outbreaks of hantavirus and Lyme disease, are epidemiological adaptations that have broadened the host ranges of those pathogens. See also: Adaptation (biology); Animal virus; Disease ecology; Hantavirus outbreak; Lyme disease; Malaria; Population ecology

Antibiotic resistance, the emergence of strains of bacteria that are resistant to antibiotic drugs, has been an important development in evolutionary epidemiology. Many diseases, including ones that had previously exhibited reduced virulence or that had almost been eradicated, have reemerged as threats to human health as a result of antibiotic resistance. The resurgence of old infectious disease scourges, such as tuberculosis, and the emergence of new ones has prompted investigators to understand the evolutionary nature of pathogens and virulence. See also: Antibiotic; Antimicrobial agents; Antimicrobial resistance; Drug resistance; Tuberculosis