I believe the closest answer would be viruses (Though viruses aren't actually living) viruses constantly change, so much that scientists don't know how many types of viruses there are in the world. For example, chicken pox is caused by a virus, however, the virus changes and becomes dormant until a stimulus of any sort activates it again and this is where you can get shingles. The reason why the immune system couldn't recognize the chicken pox virus is that it had changed and became dormant, and when the virus woke up again, it introduced a new illness, shingles.
