The very ill are humanely euthanized. The caretaker also monitors the colony for health problems, and keeps an eye out for the occasional new member, making sure it is spayed or neutered. The neighborhood has an effective, non-toxic rodent control system in place. Feral cats are afraid of humans and will run from them. You are only in danger of being bitten or scratched if you try to corner one.
Feral cats spread diseases to humans.
If there is no contact, disease will not be spread. In a sense, feral cats serve as a barrier to disease - by killing rodents. People are in danger of getting rabies from feral cats. Further, feral cats in managed colonies are vaccinated against rabies. Feral cats kill birds.