How to stop a sickness before it starts?

My boyfriend got sick yesterday. He was feeling pukey and had a headache and was shakey. We thought it was just a migraine but he is STILL feeling sick today so he might have gotten overheated or he has the flu I think. He didn't want me to hang out with him today because he was feeling so bad. What I'm worried about is me getting it because yesterday we kissed a lot and shared a drink. What can I do to keep myself from getting sick if he does have the flu?