Onychomycosis is a nail disease that causes at least half of all nail disorders. Onychomycosis is more likely to affect men than women and those over 60 years of age. People with diabetes or leukemia are also more likely to develop onychomycosis than the rest of the population due to poor blood circulation and a weakened immune system. Onychomycosis is a hard disease to treat. That is because the onychomycosis infection penetrates deep into the nails, making it difficult for topical treatments to reach them.