Forum:River's Injection

In the episode " Crash Of The Byzantium" - River gives Amy an injection of some sort, with the excuse that it is to protect her from any possible germs/virus on the alien planet. But why, in all the episodes and planets I've seen, very very rarely is anyone worried about ambient virus infections. Is this just a way of bringing the characters together to talk, or is it a definate way of taking something from or adding something to Amy's dna/blood?