What is the appropriate reaction when you encounter a snake in the wild?

When you encounter a snake in the wild, or on the sidewalk, or just outside your house, or even in your office , what should the appropriate first reaction be? How would you go through it?

I know a lot of people might panic, and I know for a fact how any sudden move might cause unwanted reactions from animals, that includes suddenly turn around and run.

I live in Florida, and have spent most of my life in Florida, and we have allot of snakes. Generally, almost all snakes will leave you alone if you leave them alone. It is very rare for a snake to be "aggressive". Most bites are, at least from the snakes perspective, defensive. There are exceptions, but they are very very rare. As for sudden movements, you certainly don't want to make a sudden movement toward the snake, especially not the head. That would probably elicit a strike. If you realized you had just put your foot right next to a rattlesnake say, move your leg away slowly and steadily, no sudden or jerky movements. If you are out of striking range and you turned and ran, the snake would probably just think you were strange and go on about it's business.

So the short answer, do nothing. Go the other way. The snake wants to meet you far less then you want to meet him.

Actually even many venom snakes' first reaction when being approached by a human or another large animal would be escape rather than strike. Some will stand their ground and try to scare you away. Their venom don't come cheap, and they want to save it for the hunt instead of wasting it on something they can't eat.

So according to you, slowly backing away is the right thing to do. What if the snake is already on the top of you? Lets say you are camping and taking a nap. You wake up to a snake passing on your legs.

It really depends on where you live. In Michigan, where I live, we only have one type of venomous snake that is somewhat common. Therefore, it is generally safe to approach a snake even if you aren't a snake expert or can't ID every species.
In other places with more venomous species, however, caution is required if you don't know your snakes. A good rule of thumb is that you should always assume it is venomous for your own safety. This does not mean you need to be afraid of the snake, but give it plenty of room to avoid you (and for you to avoid it, as many snakes can strike 2/3rds of their body length).
As was mentioned, most snakes are very non-aggressive. The few that are don't need to be feared so much as avoided/respected in their habitat. A few species may actually chase you; run, don't kill the snake. Never kill a snake for biting or attacking you. 99% of all bites are human-caused; you pick up a snake in its habitat and it may try to defend itself.
In the end: if you don't know, don't touch it. But you don't need to be paranoid, as many can be observed from a safe distance regardless.

I have never ever seen a snake in Michigan, even though I had lived there for quite some years. Although my dad had seen one once, it was swimming. My dad thought it was funny the way it was swimming.

Actually most people are afraid of snakes on sight. Many will panic if it is in close range. They would assume it is venomous.

What are some of the snake species would chase after you?

Water Moccasins/cottonmouths have been known to chase people. Most rattlesnakes will not.

I see a lot of snakes in Michigan. I am currently working in the Mojave desert, where venomous snakes are everywhere, and I have seen one, as well as one non-venomous species.

Why do they chase people?

Do they simply want to chase the people away? Or do they really want a bite that much?
It is not economical for them to waste venom on a non prey item.

Of all the moccasins I have seen, I have only been chased once. I was in a canoe. There is a theory that they protect their nesting sites after they lay their eggs. I'm not sure if that was ever proven or not. They don't actually look after their young, but there are some snakes that will protect their eggs. Many species of pythons actually "maternally incubate" their eggs, king cobras have been seen protecting their nesting site a few times, so it is the most likely scenario.