The clearest and most current example of evolution that I can think of is the modern-day mutation of viral and bacterial strains. Antibacterial resistant strains of bacteria are cropping up in hospitals across the industrialized world, where we overuse antibacterial washes and such. The chemicals kill most of the bacteria on your hands, but a few hardy ones manage to survive. They pass on their hardy traits to make a new population of hardier (resistant) bacteria.
The avian flu (H5N1) rampant in Asia is another example. On the one hand, some strains of it are mutating to become resistant, which means they will come to dominate their weaker peers. And on the other hand authorities worry that a strain transmissible from human to human will develop. This would be devastating to humankind, as 150 million could die (around 2.3% of the 6.5 billion in the world), and of course it'd start with third world countries, and make it to poor people in first world countries (though America's politicians would be safe).
An optimally evolved human disease would be one that could live in its host for years, and spread quickly and easily before its host died or was cured. Common cold spreads easily, but we fight it off to quickly. HIV can't be gotten our of our systems, but it takes too much to spreads it. There's a large number of bacteria commonly found on the skin that I think fit the bill the best, as they live happily on our skin and most people don't know about it at all. Things like staph, and the stuff that causes ringworm and athlete's foot. Those're there all the time. Yum.