‘Kill or be killed’ shows how viruses survive—they must reproduce before the host either dies or its immune system recognizes and eliminates them. The transmission routes of viruses such as flu, measles, common cold, herpes simplex virus, HIV, Epstein–Barr, and hepatitis B are discussed. How do we fight viruses? All living organisms have defences against invading viruses. Vertebrates, and possibly some invertebrates, are immune to re-infection by the same virus. Another protective mechanism, used by plants, but also by insects and other animal species, is gene silencing by RNA interference. The human immune response is explained, discussing the role of lymphocytes and immunopathology, where the immune response may actually harm the body.