Because when it comes to public health, individual action only gets you so far. It doesn’t matter how good your water is, if your neighbour’s water gives him cholera, there’s a good chance you’ll get cholera, too. And even if you stay healthy, you’re not going to have a very good time of it when everyone else in your country is striken and has taken to their beds.If you discovered that your government was hoarding information about water-borne parasites instead of trying to eradicate them; if you discovered that they were more interested in weaponising typhus than they were in curing it, you would demand that your government treat your water-supply with the gravitas and seriousness that it is due.The public health analogy is suprisingly apt here. The public health threat-model is in a state of continuous flux, because our well-being is under continuous, deliberate attack from pathogens for whom we are, at best, host organisms, and at worst, dinner. Evolution drives these organisms to a continuously shifting array of tactics to slide past our defenses.Public health isn’t just about pathogens, either – its thorniest problems are about human behaviour and social policy. HIV is a blood-borne disease, but disrupting its spread requires changes to our attitudes about sex, pharmaceutical patents, drugs policy and harm minimisation. Almost everything interesting about HIV is too big to fit on a microscope slide.