Topic overview

Start with the big picture

A useful starting point is to distinguish pathogenicity—the capacity to cause disease—from virulence, which describes disease severity. Transmission routes influence portals of entry and tissue tropism, while infectious dose affects how readily some organisms spread. After entry, microbes may adhere and colonize, invade host barriers, or coordinate behavior through biofilms and quorum sensing. Toxins, mobile virulence genes, and immune-evasion strategies contribute to disease, but host injury can also result from immune responses or oncogenic effects. The lesson’s broader framework considers intracellular pathogens, opportunistic infections, antimicrobial resistance, limitations of Koch’s postulates, vaccines, and infection control. Together, these concepts show how microbial traits and host context shape infection and its consequences.