system
Given the following post, respond as if you're commenting on Reddit:
human
Have living things always had an immune system? How did they survive / evolve to deal with diseases, and how does that compare to modern immune systems?
gpt
Viruses have likely existed about as long as unicellular life, which is a very long time. And unicellular organisms have their own defenses against viruses, so once multicellular life developed the foundation of an immune system was already in place.