Hepatitis B is a very serious viral illness of the liver that frequently leads to cirrhosis and even death. It's transmitted through body fluids and sexual contact and is very common in many countries. For example, one doctor in Fiji told me that half of all children there are infected by age 18. There is a vaccine available against this infection, and it's recommended for anyone spending significant periods of time in endemic areas. No vaccine or medication is 100 percent effective, so it's important to understand how diseases are transmitted in order to limit your exposure.
Dive travelers are most at risk from three sources.
After a few days, not only Montezuma, but all of his extended family have been exacting their revenge on your intestines. This too shall pass. Traveler's diarrhea is the most common ailment afflicting divers, and it's caused by food and water containing bacteria different from those your body is accustomed to. While traveler's diarrhea is a self-limiting ailment, there are other diseases and parasites that you can get from consuming contaminated food and water that aren't so benign. These include cholera, typhoid, hepatitis A and amoebic dysentery.