Science fiction could scarcely have dreamt up anything stranger than the virions. But these invaders are real enough; we have all suffered their attacks. They are the whole family of viruses, from smallpox and rabies down to influenza and the common cold.
Where do the invaders come from; and how do they succeed in taking us over so completely? Why are they so difficult to fight? How is it that we never build up an immunity to 'flu? How do some of the virions manage to hide away in the body, only to re-emerge later? And faced with such subtle and devious attackers, is there anything we can do to resist? Narrator Paul Vaughan