Overview: What's the purpose of sex? To protect us from disease! Brian Leith unravels the intricate puzzle of sex and finds that: Males seem to be a waste of time and energy - many of the successful plants and animals (like the weeds in your garden) do without them altogether! Big animals - from polar bears to human beings - always have males as well as females - for a very good reason! The vital clue to the mystery lies in parasitism and infection. Out of the puzzle emerges a sparkling new theory of sex that looks set to keep biologists buzzing for years.