Despite the fact that polio has been virtually eradicated from many western countries, it is still counted as an incurable disease. Polio is a pretty nasty viral infection of the nervous system. It will produce the usual flu-like symptoms, but can also have other devastating effects. Polio can bring about the paralysis and deformity of the limbs, throat and chest, making it artificial respiration necessary. We have managed to get polio under control in a lot of countries through the use of vaccinations, so it is preventable, but if you do become infected (if, for example, you don't vaccinate) then there is no treatment or cure. The trouble is that polio is a viral infection, and viruses are notoriously tricky to nail. Compared to bacteria, viruses are incredibly simple. Far from this making them easy to kill, it means that they have fewer weaknesses to target. Whilst bacteria are living cells that replicate and metabolise, viruses hijack the processes of other living cells (i.e. yours) in order to survive and reproduce, which helps them to evade the immune system by essentially "hiding out" in your cells. They can also mutate and develop a resistance to treatments amazingly quickly, so even if you do find a treatment, it won't be effective for long. Basically, the best approach with diseases such as polio is preventions (vaccination), not cure.
