excellent question pixelator! There is a very good advantage of a disease that results from a genetic adaptation that provides a survival advantage. The disease is called Sickle Cell and its a disorder of red blood cells which carry oxygen from our lungs around the body. Red blood cells in people with Sickle Cell are mishaped and not flexible, as a result these people suffer from anaemia and have a short life expectancy. Sickle Cell is caused by a mutation in the haemoglobin gene, we all carry two copies of every gene including haemoglobin. If someone inherits two copies of the Sickle Cell gene from their parents then they get the disease, but if they only inherit one mutant gene then they have no disease. It turns out that the mutation in haemoglobin protects against malaria. So in countries where there is a lot of malaria there is a selective advantage for the Sickle Cell gene.
Some runners using prostheses instead of their amputated limbs have been found to run faster than healthy athletes. But wait – even this turned out to their disadvantage: Disqualification for the Olympic Games for being faster than allowed.
On a different note, Temple Grandin — a famous researcher in animal psychology — may owe some of her success to autism. She has just been named one of the TIME 100 most influential people in the world, and you can watch a talk by her here.
Diseases can be other forms of life that choose to live in our body. Some bacteria don’t get on with our bodies and some live happily in our stomach or nose. So some bacteria have advantages like digestion for example. However, if we know of it as a disease, it is because we don’t like having it and it therefore has no advantages for us as an individual!
Other diseases are genetic and are problems with parts of our body. I can’t see any advantage in that, other than if genes did not mutate then we would never evolve, but that is of no comfort to any individual.
Of course, if you have a disease you develop a resistance to it so that could be thought of as a good thing. But generally, no, I don’t think so!