When to get vaccinated: At least a month before your trip

Hepatitis A and hepatitis B are two very different diseases, both of which can hurt the liver, but there’s a combined hep A/hep B vaccine that you can get if you haven’t been immunized against either one (or just don’t know what you’ve been vaxxed for in the past). You can get this on an expedited schedule too, if you’re up against the clock: Typically this shot is given over the course of six months, but in a pinch, three doses can be administered in 21 days. There are also shots specifically for hep A and hep B, respectively, if you know you’ve been vaccinated for one, but not the other.

In terms of what these diseases are, let’s start with hepatitis A. It’s a very contagious liver infection that’s transmitted through contaminated food and water or contact with an infected person. The symptoms, such as fatigue, nausea, and jaundice, can last up to two months (and, for some unlucky people, up to six months). Pretty much every country in the world recommends travelers get this shot (which, by the way, requires two doses given six months apart, but even one dose provides strong protection). “It is the most recommended vaccine for travel,” Dr. Sammon says.

Hepatitis B is a much more serious liver infection that can be deadly. It’s spread through bodily fluids (like blood, semen, or vaginal fluids), most commonly through sexual contact with someone who’s infected (there are an estimated 350 million hepatitis B carriers globally). It can also be transmitted through sharing personal care items (like a razor or toothbrush) or getting a tattoo or piercing from unsterilized equipment.

Hep B is not as common as hep A, but it’s much more serious. Though many people will get better in a few weeks, some will get chronic hepatitis B, which can lead to issues like liver failure or cancer. It’s particularly important for people who expect to have any sexual contact or medical procedures abroad to get vaccinated for hep B first, Dr. Sammon adds, so talk to your doctor to gauge your risk—but no matter what, “I would recommend that you get it if it’s on the CDC list for your destination,” she says. The good news: Hepatitis B is entirely preventable by vaccines.

Typhoid fever

Recommended if you’re traveling to: Eastern and southern Asia (especially Pakistan, India, and Bangladesh), Africa, the Caribbean, Central and South America, and the Middle East

When to get vaccinated: At least two weeks before your trip

Typhoid fever is a bacterial infection that causes a sky-high fever along with weakness, headaches, and gastrointestinal issues like diarrhea and constipation. If you’re treated early, you can recover swiftly, but there’s one issue: The infection is often not recognized quickly because it resembles other common illnesses, says Dr. Sammon, adding that it can be fatal.