I don't do anything. I suspect the andrenaline or whatnot from traveling keeps the germs at bay. I can only once remember being severely ill on a trip from the flu, and it was in 1997. Everybody had that one. Can't remember numbers any more but I think 40,000 people died? Most of them older though. Not travelers.
OK, I googled it just now and learned that there were 50 million cases of flu and about 47,000 deaths per year from the flu in the United States each year in the 1990s. I just remember 1997 because I was really sick from that one. 40,000 deaths wouldn't be any unusual number apparently. Well, we've all got to go sometime, I guess.