Odds of Hanging On
The cool thing about being a specialist in something is that you're, well, special. Just don't let that go to your head because there's nothing worse than an aspiring epidemiologist with rampant narcissism. Nothing, we tell you, nothing.
Once you're a true-blue epidemic academic, you're not going anywhere unless you want to. And even then, you're probably going to have difficulty leaving considering how specialized you are. No, the burger joint is not accepting applications from PhD holders.
If you have a real passion for what you do, and find yourself with a drive to help the rest of us poor zombie breakfast sandwiches avoid a life of diseased terribleness, then you're going to have no trouble staying in it for the long haul.