[Symmetry]

Have you ever seen that sort of generalization in an article on Hacker News? Even when you have situations like, e.g. nobody has ever recorded a case of a person with a certain genotype being infected by a norovirus, you don't see papers claiming that its impossible.

[Goladus]

Yes. Realize that often the generalizations are a lot more subtle than your example, and of course anecdotes aren't always the best or only counter-arguments you'll see, even if they are valid. Start looking and I am sure you will find examples.