Why We Fall Ill

Having spent a track on microbes, it would be easy to leave with the impression that being ill and being infected are the same thing, so this track starts by separating them: a disease is infectious only when its cause can travel from one host to another, which scurvy and an inherited condition never can, and which says nothing at all about how serious the disease is. What follows is the journey that makes the other half infectious, drawn as six steps — leaving a host, surviving the outside, reaching somebody, getting past a defence, multiplying — because a chain with six links is a chain with six places to break it, and almost every control measure ever devised attacks one named link. The last three tutorials go inside. A cut lets bacteria past the skin and several consequences start at once rather than in a line, which is why the redness and swelling of inflammation are the response rather than the damage. The white cells that arrive turn out to be two kinds with opposite weaknesses: one ready immediately and effective against anything, one useless for days and then decisive against exactly one pathogen. And the track closes on why the slow route is worth having, which is that it is slow only the first time — the fact a vaccine exists to exploit.