What do you remember best from your last vacation? The tropical beaches? The historical ruins you toured? The incredible new wildlife you saw? Or are your memories of the food poisoning you got from that first dinner? Or of spending the night in a strange emergency room after spraining your ankle?

Nothing can ruin a vacation faster than getting sick or injured. Here's a look at the six most terrifying travel risks-and how you can prevent them.

1. Diarrhea: Sometimes termed turista or Montezuma's revenge, it's common for travelers visiting foreign countries to feel ill after drinking the local tap water or eating food prepared with poorly sanitized water. As many as half of all international vacationers will develop diarrhea (usually during the first week of their trips), according to the Centers for Disease Control and Prevention (CDC). The best way to prevent it is to drink only bottled water or boil local tap water. Also beware of fruits and vegetables that may have been rinsed with local water.

2. Blood clots: Sitting still for long flights, like those to Europe, Asia, and Australia, can cause a dangerous condition known as deep vein thrombosis (DVT). It develops when a blood clot forms in the leg or thigh and, in very serious cases, travels through the bloodstream, possibly getting stuck in the brain, lungs, or heart, according to the National Institutes of Health (NIH). People with high blood pressure and diabetes are at greatest risk. One way to help prevent it is by walking, even very short distances up and down the plane aisles, before and during the flight. The Medical College of Wisconsin recommends people at risk talk to their doctors about making adjustments to their medications before taking a long flight. Taking a low-dose aspirin before traveling may help, too.

3. Mosquito bites: Beyond the annoying itch that mosquito bites cause, these little insects can infect travelers with malaria, too. Malaria is a flu-like disease that is most common in Africa, Central and South America, parts of Asia, and Eastern Europe. According to the American Academy of Family Physicians, people traveling to high-risk locales can protect themselves by using insect repellents with DEET, wearing clothes coated with permethrin, and using mosquito netting over the bed. Some travelers may want to ask their doctors about getting preventative malaria medicine.

4. Dehydration: Dehydration can begin on the way to your destination, and it can be aggravated during your trip if you spend too much time in the sun, drink too much alcohol, or get food poisoning or diarrhea. Fortunately, you can prevent dehydration by drinking plenty of fluids. Water is sufficient, and the CDC recommends avoiding alcoholic drinks and beverages with too much sugar, which could cause the loss of body fluids.

5. Waterborne infections: It's possible to pick up parasites, such as schistosomiasis (which can cause serious infections) or Naegleria (which can cause severe brain infections), by wading or swimming in fresh water lakes and streams. Travelers should definitely stay away from warm, stagnant water and areas with signs warning them to stay out of the water.

6. Accidents: The State Department estimates that more than 200 American citizens die in traffic accidents abroad each year. Remember, other counties have different safety laws than the United States. For example, cars may lack seatbelts, or boats may not be required to keep life jackets on board. If you're driving a rented vehicle, research the local rules before you get there: Which side of the road do they drive on? Who has the right-of-way at traffic circles? Is it customary to honk when going around a sharp bend?