Don’t shave with a rusty razor. As others have said rusty things make have bacteria on them and while things outside like nails will have the bacteria that causes tetanus razors can have other bacteria.

In 2007 at The Excalibur, in Las Vegas, NV. A Las Vegas a woman shaved her legs with a rusty razor blade and got a Group A streptococcal infection, which develops into necrotizing fasciitis ("flesh-eating" bacteria), when she cut herself. The bacteria in her blood stream broke through a wart in her face and started eating away at her face, eventually killing her from sepsis.

Throw out rusty razors.