Stress
Research itself can be frustrating, as anyone who's been forced to write an essay can tell you. (Someone like you, probably. Go ahead, tell yourself. We'll wait.) It can be even more frustrating when you know that lives are at stake, and you're making what feels like inches of progress on the marathon that is fighting cancer.
Sometimes, developing a therapy or treatment or finding out something that we didn't know before can take years or even decades. It's a long battle, so be ready for the occasional less-than-stellar day.
There are also the physical issues that come with living this lifestyle. There will be long hours, working weekends, late-night study sessions, conferences, board meetings, committee meetings, lunch meetings, and even the occasional work-related trust building seminar. On the bright side, if you work at a big university, there's got to be a physiotherapy class going on somewhere on campus.
The important thing to remember is that, no matter what you do in the regular day-to-day, the mere fact that you are researching, discovering, and working towards a cancer-free future means that you're making a difference. Cancer is never going to give up, and neither should you.