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The most common venomous snakes in the United States are pit vipers. Pit vipers have triangular heads, elliptic (slit-shaped) pupils and fangs that are folded against the upper jaw while the snake is resting. The fangs rotate down and forward when the snake bites. The snake's name comes from the depression, or pit, midway between the eye and nostril on either side of the head. This pit is a special organ that senses temperature, helping the snake locate its prey. There are three main groups in this family: rattlesnakes, copperheads, and water moccasins (also called cottonmouths).

A snake can control the amount of venom it injects. In 50 to 70 percent of bites, the snake does NOT inject venom. It is not possible to tell immediately if a snake has injected venom, so all snakebites should be treated as an emergency. Even if no poison was injected, the bite can become infected with bacteria that can cause tetanus, cellulitis, or gangrene.

All venomous snakes are able to inject poison from birth and should be treated as dangerous. The venom of a small or immature snake may be even more concentrated than that of larger ones. A dead snake can bite for up to an hour after death from reflex.

The eastern coral snake is found in the Southeastern United States and northern Mexico. It is a burrowing snake that lives in pine woods, near ponds or streams, under leaves or mulch, and in logs or stumps. The western coral snake is rare and is found in the Southwestern United States.

If you know what the snake looked like, it may help determine whether it was venomous. How many times the snake bit and how the bites felt also can help identify the snake; that information also can indicate the amount of venom that may have been injected. It is also important to remember such symptoms as numbness, nausea, or tingling around the mouth; a metallic taste in the mouth; muscle cramps; trouble breathing; and dizziness. None or all of these symptoms may occur. Seek medical attention as soon as possible. Call 911 or your local emergency medical services.

The bite of a rattlesnake looks like two puncture wounds.

The bite of the coral snake usually looks like scratch marks and is easy to miss.

Generally speaking, the venom from a rattlesnake causes swelling around the area of the bite; if a large amount of venom is injected, other symptoms can occur. The amount of swelling depends on the amount of venom injected. Victims who have small body mass (such as children or elderly) or heart, kidney, or lung problems will be less able to cope with the venom from any snakebite. Bites on the trunk, neck, and face are more serious because the venom can spread faster through the bloodstream. Most bites occur on the arms and legs. Bites that puncture a blood vessel can cause death quickly.

These aggressive snakes are native to the Southeastern United States. They live near water. Their venom is not as potent as the rattlesnake, cobra, or coral snake, but their bite should be considered serious.

In recent years, the advice for first aid for snakebites has changed. Here are some dos and don'ts from the Centers for Disease Control and Prevention (CDC):

• Do remove jewelry on the affected limb.
• Do notice the time of the bite and the time symptoms develop.
• Do keep the bitten limb below the heart.
• Do get to a hospital or other medical help as soon as possible.
• Do try and remember the color and shape of the snake.
• DO NOT attempt to trap the snake or delay medical treatment attempting to identify the snake.
• DO NOT apply a tourniquet, because it could stop arterial blood flow and cause more tissue damage.
• DO NOT apply cold compresses or ice, because this treatment could cause tissue damage.
• DO NOT cut into the site of a bite with a knife or razor.
• DO NOT try to suction the venom by sucking on the wound with your mouth.
• DO NOT give the victim anything to eat or drink.

The best advice when you see a snake, the FDA says, is to leave it alone. Don't pick it up, try to get a closer look at it, or kill it. When hiking, stay on trails or paths. Be careful where you place your feet and hands, particularly around rocks and wood debris.