It’s the classic Red Queen argument. We adapt to fight off a virus, the virus adapts to infect us better. It keeps the two organisms in a kind of equilibrium.
Here’s some more information about the Red Queen Principle.
Also, Evolution can only work with what it’s got to work with. If I don’t have yarn or knitting needles, for example, I can’t knit a sweater. For evolution to “work toward” making us free of illness, a person (preferably a good many people) must somehow mutate into being disease-free. Then, those people must outbreed the people left who still get sick. This works best at population bottlenecks, where the better-off survivors can only breed with each other and fill up the vacancy. As it is, there’s a whole heaping lot of people around who are all breeding like crazy, and I bet that it’s too much for a theoretical pair of disease-free mutants to tackle – especially in the short run.
There are cases of people adapting to defeat certain illnesses, though! For example, there is a subgroup of people of Northern European descent who are naturally immune to the AIDS virus. If AIDS took out everyone else, these people would live and breed and the resulting generations would be largely free of AIDS. Also, my blood type is AB, which makes me part of the lucky small percent of people that are resistant to cholera.
But… the point of being ill? It has very little to do with people, actually. It’s the bugs’ way of making their living. We just happen to be convenient bags of nutrients.