who are the puppet masters
The Puppet Masters- Robert A. Heinlein 1951
Adopting biological warfare, the authorities culture Nine-day Fever and its cure in bulk sufficient to cover the country, and then infected slugs are allowed to escape into the heavily infested areas. Several days later thousands of medics are landed in those areas to give the cure to those people whose slugs have died…
https://en.wikipedia.org/wiki/The_Puppet_Masters