In the United States, the State Department issues travel advisories that relate to health threats and political unrest. Its history dates back to July 21, 1789, when the Senate and the House of Representatives approved the formation of the Department of Foreign Affairs. Since the State Department is responsible for the assistance and protection of American citizens who are either traveling or living overseas, it must be diligent about evaluating the safety of a given destination. As such, there are members of the State Department who live in overseas locations, giving them first-hand information about the health and political climate of the country. If you visit the State Department website, you can find a list of countries that have either political or health advisories.
While the State Department's travel advisory page can advise you about health threats in various countries, the Center for Disease Control (CDC) has more detailed information. Visit the CDC website for up-to-date information about outbreaks in specific countries, drinking water safety and required vaccinations.
In addition to advisories sent out by the State Department and the Center for Disease Control, the Transportation Safety Administration (TSA) will issue warnings about airport safety. This government agency was created as a result of the September 11 terrorist attacks. The Aviation Security Advisory Committee is responsible for making recommendations about airport security measures. Since these measures seem to change on a regular basis, it's a good idea to check the TSA website before embarking on your trip.
Keep in mind that government agencies will usually be extremely conservative in their recommendations. In general, they will err on the side of caution, so if there's even a small possibility of a political or health threat, they may issue a travel advisory against the destination. However, despite the warnings, you might hear of friends who have visited these locations, and had a safe and wonderful experience. It all comes down to common sense. Know the local customs and respect the health concerns. For example, many governments have issues with tourists who take photos of political demonstrations. Doing so can get you thrown in jail. Likewise, you can remember to use bottled water in a country whose water has been deemed unsafe, but forget to avoid the ice cubes in your soda.
When you go to the State Department's Travel page, you might find information about visa requirements. This is a confusing issue for many people. For example, your British friend may not require a visa to visit a specific country, but you do. The answer lies in a concept called reciprocity. For example, if the United States charges a visa fee for citizens of a given country, that country will apply a fee of the same amount to U.S. citizens. The visa requirements are an important part of the travel advisory. Many people fail to research this information, and are unpleasantly surprised when they reach their destination.