And some viruses are worse than others. Some will hide out in your spinal cord forever and make an appearance when you're under stress. If you've ever had to watch someone go through an outbreak of shingles... holy crap that looks miserable. I had Mono when I was 15 and still get what I take to be occasional, minor resurgences of that level of total body-wide exhaustion, every five years or so for a couple days. A friend who had mono confirmed that he experienced the same. It's a particular, very identifiable feeling throughout your body - not too different from the feeling after the second and third covid shots.