Yep, a random one, and one where a) you take damage before the body figures it out, and b) it's pretty random which protein the immune system manages to identify (basically: The first one which identifies the virus), which may as well be one which changes widely so that even if you get immunity to your current infection you may not be immune to the variant someone else harbors. Whereas vaccines try to expose the body to the most stable proteins, for longer protection.