Tarantulas are fairly hardy animals that don't have a lot of illnesses that are natural to them. Their lifespan in captivity for a female is easily into the 20's for those desert species, and that's a testament to the fact that they are hardy creatures. However, as the keeper you've got to make sure that you give it the basic care. The temperature and humidity in the cage is paramount, as is giving it clean fresh water at all times.
A dehydrated tarantula, is probably the common mistake that I see, when I see peoples tarantulas, where they bring them in and say, "there's something wrong with them." Nutritious, gut fed crickets, if that what you're going to feed them. If not, a variety of different insects: goliath worms, butter worms; which are high in calcium, waxworms; which are fatty and will put fat on the animal. It's good to give the animal variety, to be the spice of life. But, if you see that suddenly the animal is not performing, he's getting a little sluggish, and certainly if he starts to curl up his legs a little bit, normally that's a sign of dehydration, which I guess is an illness, but really it is a keeper illness.
You didn't do the right job. You're not giving him enough water. So, we can rehab tarantulas that do that. That little bowl that I had here, which I've used today. The way we do it, we can take the tarantula, and just put his face, his fangs, right in the water. Fill it up to the top and let him just sit there. They have like a vacuum mouth. They suck in moisture. So, that is a way to replenish the bodily fluids inside of the animal, and that's very very effective. You'll see if the tarantula's stopped and he starts to curl, that's the best method.
If there's something beyond that - he's not eating or dehydration didn't work, you might have to take it to a specialist veterinarian to see what else it's going to be. Usually it's not illness. I find that most of those sick tarantulas are really keepers making mistakes. Too high, too low of a temperature, wrong humidity, not giving it the proper foods in the proper times in the proper amounts, and most of all water and fighting dehydration. So, if you follow those guidelines and do your homework, you should have an animal, if you get a female that's good for 15 to 25 years.