There's no federal entitlement to being paid if you're sick, so companies come up with their own policies.
So companies often have a strange concept of "sick days", a specific number of days a year you're allowed to be sick. If you're sick more than that you have to use your vacation days, or unpaid leave when you're sick.
(And of course American companies often have weirdness around vacations too. More so in companies where there is allegedly "unlimited time off". But that's kinda off-topic now.)