A frequent claim made by religious people is that "God cured my medical condition...god healed my grandmother of cancer when the doctors said there was no hope...", or some other variation of this. When faced with this claim, a number of questions need to be immediately asked.
First off, how does one differentiate between a healing that god is responsible for versus a healing that the immune system or medication is responsible for? If you're a religious believer, is god given credit for every healing that occurs, whether it's cancer, the common cold, or strep throat? Clearly not, because even the most scientifically-ignorant creationists understand that the immune system and medication play some role in combating illness.
Perhaps we give God credit when prayer is involved?
If that's the case, how do we determine what was responsible for the healing when a person takes medication, follows the advice of their doctors, has a functioning immune system, and prays for good health? As Jeff Dee once said on The Atheist Experience, when somebody says a prayer and takes an aspirin, which one cured the headache?
It's also worth asking why god is only able to cure conditions that people—including nonbelievers—can recover from naturally or with proper treatment, like cancer. A truly impressive display of supernatural intervention would be a person with a missing limb saying a prayer, and having the limb spontaneously regenerate.
Why is it that the only things which god allegedly heals are things that god isn't necessarily required to heal?
The answer, of course, is that god has no hand in these sorts of things. Instead, what we're seeing is selection-bias: people have a health issue, they pray about it, and if they recover, they attribute it to god. If they don't recover, and if they instead die a miserable death, even while uttering prayers up until their last painful breath, they or their religious peers will almost never count this as evidence against god's existence.
Instead, they confabulate a number of expalanations that allows them to maintain their faith: "God works in mysterious ways"...or..."I'm being punished for my sins."
Simply put, religious people count the hits and forget the misses when it comes to god healing their medical conditions.