Concept preview

What you will be able to do

1. 01Describe the main routes by which viruses spread between hosts.
2. 02Explain how viral infection causes symptoms of disease.
3. 03Explain how the immune system and vaccines protect against viruses.
4. 04Explain why viruses are difficult to treat and why some evolve resistance.

A viral disease begins with a single infected cell and, unchecked, can end with a global pandemic. Between those extremes lies the whole story of how viruses spread, harm us, and are fought off. Because a virus cannot move on its own, it depends entirely on being transmitted from host to host — and the more easily it spreads, the more successful it is. Some of history's deadliest events, from the 1918 influenza pandemic to COVID-19, come down to a virus that combined efficient transmission with a body unprepared to fight it. This topic is about both halves of that contest.