Ecology and evolution of virulence.

Parasites - from microscopic viruses and bacteria to eukaryotic organisms like helminths - depend on their hosts to survive and reproduce. However, by exploiting their hosts, parasites cause a degree of harm. This harm or 'virulence' can be measured as host death, a reduction in host reproductive output, or other changes in host traits reflecting disease severity (for example, reduced host red blood cell counts during malaria). Why would parasites harm their home, and why are some more virulent than others? In this Primer, we explore how virulence arises, why it changes across environments and evolutionary timescales, and what these patterns reveal about the biology of infectious diseases. Virulence is more than a measure of harm - it's a window into the rules of life. Understanding the ecological and evolutionary factors that underlie virulence helps explain how hosts and parasites coexist, how virulence traits emerge and persist, and how ecosystems are shaped by the push and pull of conflict and cooperation.