Being ill is a part of everyday life, and being on the move does not free them from that fact, especially when traveling by themselves. The key to minimizing the chances of this, however, is to combat the risk factors of getting sick at all actively. The last thing you want to do is risk yourself more than you have to.
Between recycled air, questionable meals, and jet lag, travelers often find that their immune system is down for census, making their journey miserable. And while handwashing can help limit some of the germs that you encounter on your travels, there are only so many sinks and some soap can. Prevention is much better than any cure.