Cancer

It only takes one rogue cell to cause cancer. All cells are programmed to kill themselves if they detect a problem within their systems. If they sense a virus or mutation, they destroy themselves completely. But some cells develop a mutation that cancels out that defense mechanism, which is called Apoptosis. Of the 100 billion cells created in the human body each day, at least 100 thousand mutate and perform apoptosis.

At first, a cell that mutates so as not to perform apoptosis will be harmless. Through mitosis it would replicate itself and it's copies would have that same mutation. But eventually, one of those cells would form it's own mutation, and because of the original cell's mutation, it wouldn't kill itself. The new mutation might be something like replicating itself faster than normal. At this point the cells are still harmless. The cell with two mutations would continue to replicate itself. But the odds of mutation in cells are high, and without apoptosis, the mutations wouldn't be dealt with. Soon cells might develop a mutation causing them to secrete acids and waste that kill the cells around them, or maybe it would be a sensory mutation that makes the cell think that other cells are viruses. Mutated cells can even cause marks or rashes on the skin.

Mutated cells are called cancer cells, and you can imagine that they would be very hard to eliminate. It would be difficult for a drug to seek out the mutated cells and not kill the healthy one. For a virus medicines and drugs would simply kill all organisms in that body that match a viruses description. For cancer cells, a cure would need to cleanse the body of mutated cells that don't match any description.