If your employer doesn't have dedicated sick pay, then why not? Perhaps they should? A few days where >50% of their employees are off sick due to the flu of the month going around ought to convince them otherwise.
This is why most countries (by "unheard of in some countries", I think you mean "unheard of in the US") have sick leave in some form -- it's both recuperation time and voluntary quarantine.
Actually, by some countries I meant East Asia. I had a friend get fired after getting hit by a car and needing 2 months leave (because his office had no wheelchair access). Technically not legal but you can't exactly fight a well funded employer when you are broke from medical bills.
It is quite common to see very sick people working. People who probably should be in hospital, not just on leave.
This is why most countries (by "unheard of in some countries", I think you mean "unheard of in the US") have sick leave in some form -- it's both recuperation time and voluntary quarantine.