> The ants carry out prophylactic amputations. This not only protects the colony from infection but also doubles the survival rate of the injured workers.

To keep everybody around you healthy makes the probability of caching a disease lower for yourself, too.

Grooming behaviour in primates helps in the same way. And it is so important that it is linked to all kinds of mental rewards.

To let disease run amok in your own neighborhood it would be very costly.