That's fair, but the thing about anecdotes is they are just anecdotes. They are good at telling us that something can happen, but to truly know how often it happens we need data.
Many "serious" diseases you might think of such as polio are caused by viruses that for vast majority of people result in asymptomatic or very mild illness. Like for 95% of kids polio is nothing. Imagine 5% over and over again though now you're playing a game of russian roulette and the odds keep stacking against you, especially if say there is a little bit of heart inflammation on one, some lung fibrosis on another. It starts to snowball.