Physical Danger
The point of international aid work is that its caring, idealistic adherents (that'd be you) get sent to places where there are problems. Those problems could involve disease, poverty, or starvation. Those problems could involve instability, violence, or outright war.
Let's look at some worse-case scenarios. In 2012, according to the Aid Worker Security Database, 274 aid workers were killed, kidnapped, or seriously wounded. While international aid workers were far less likely to face violence than aid workers who hailed from the country where the violence occurred, the fact is that you could get assigned to a place where bombs are dropping and bullets are flying, or where people view you as a legitimate target.
And, if a rusty machete doesn't get you, there's always the chance that the germs or parasites will. Half the world's population is at risk for mosquito bites that lead to malaria…and guess where all of those people live? That's right, in countries where you'll be working…which means you're one of the at-risk, too!
There's also schistosomiasis, a tropical disease caused by parasites that live in contaminated freshwater. There are the lovely hemorrhagic fevers, like Ebola and Marburg. And, if you really want to gross yourself out, go look up the damage a botfly in the brain can cause. Yuck.
The good news is that you can mitigate your risk of dying in a hail of bullets by choosing to work for organizations that don't send their people into harm's way. The not-so-good news is that viruses, bacteria, and parasites are much harder to escape.