I once saw a statistic that showed that cats who lived primarily outside have an average life-span of 11-12 years, while indoor-only cats lived about 15-16 years. I can definitely believe this. Over the years I have seen numerous situations of cats who went outside getting injuries and illnesses that you would never see if they had stayed inside. Many of these injuries are repeated ones, as cats get into fights again and again. Bite wounds, fractures, poisonings, and similar problems are relatively common in outside cats, as well as serious and incurable infections such as feline leukemia and feline AIDS.
Today was another example of it. The cat was allowed outside whenever he wanted. Earlier this year we had treated him for an infection on his leg, and this wasn't even the first time. Today we saw him again, with another bad wound. Most of his left front leg was affected, requiring sedation, disinfecting, and removal of the dead tissue. The skin was completely gone over the infected area, and there was a lot of brusing and damage. This was likely caused by a bite from another cat that abscessed and then ruptured. Here is a picture of what it looked like after being cleaned up. Sorry that it's slightly unfocused, but I didn't have my camera at work and had one of my nurses take a picture with her cell phone.
I bandaged the wound, and we will have to change the bandage every other day for a few weeks. With proper treatment this should heal well, though it's going to take a while and will likely leave a sizeable scar. If the owner isn't compliant with coming back in for rechecks, this could turn much worse and not only take longer to heal, but potentially spread the diseased tissue and require surgery to correct. This kind of problem is common with cat bites. The cat's teeth penetrate the skin, and usually pull out through the same hole. This is a rather small hole that heals closed quickly. However, a cat's mouth is full of bacteria, so this is kind of like injecting a load of bacteria under the skin and then keeping it from draining out. Infection builds and festers until it suddenly ruptures. Often the initial wound goes unnoticed by the owner, only becoming obvious when it opens like in the photo.
So those of you with cats who go outside, keep this in mind and remember this picture. This is not an unusual occurrence. Cats who stay inside aren't mentally harmed, are exposed to fewer health risks, and on average live longer lives.