There is a theory that colds and flu and many sexually transmitted diseases also affect behaviour enough to encourage more spread. In other words, it's possible that being infected with an STD makes you more sexually active.

It's a theory as it's incredibly hard to do studies about. The main study this was based on was done on people with the flu - they used students who had the flu jab (mild flu effects etc) - and saw that those students went out more and socialised. Subsequent studies think this might be because they felt more protected being jabbed, rather than the virus affecting their behaviour.