A viper is a venomous snake from the family Viperidae; such snakes are also known as viperids. Vipers are among the most famous of venomous snakes, probably because their population is quite widespread and because the snakes can deal a hefty bite when provoked. Like most venomous snakes, vipers would actually prefer to avoid human interactions, if possible, so if people remain calm when encountering vipers, they are much less likely to be injured.
Several features distinguish members of the viper family. The snakes tend to have very heavy bodies, with distinctively triangular heads. Many vipers are also quite brightly colored, which can be a useful alert for people who wish to avoid them, and they are very fast-moving snakes, with the ability to open their mouths quite wide. The speed of the viper is probably the most dangerous thing about it, as people can be struck by vipers before they even realize they've encountered a snake.
The most famous feature of the viper is its hollow fangs, which fold back against the roof of the mouth when not in use. Vipers are also capable of controlling the amount of venom they inject into prey, making a calculated decision which is based on the size of the prey. Since most vipers strike to disable, rather than to kill, their ability to control the amount of venom they exude can be quite useful, as it allows them to conserve their venom for future use.
Vipers can be found in Asia, Africa, Europe, and the Americas, and there are several viper subfamilies, including the Crotalinae, or pit vipers. As a general rule, people who visit areas with a large viper population will be warned about the snakes, and many such regions also have extensive stocks of medical supplies to treat viper bites. If you are traveling in an area where vipers are known to be an issue, you should not stray off trails and roads, and you should be very careful in warm weather, when the snakes may be sunning themselves; vipers also like to lie on paths and roadways at night to absorb radiated heat.
When traveling in snake territory, heavy pants and boots should be worn, so that if a snake strikes, it has less chance of reaching your skin. It's also an excellent idea to make noise as you travel to alert snakes to the fact that you are coming, and watch where you put your hands and feet. If you do see a snake, back away slowly, and in the event that someone is bitten, the victim should be kept as still as possible until a medical crew arrives.