If you can't get your doctor to write you a letter you could use to receive disability payments your best bet for the moment may be to return and sign up for Social Security benefits. You are old enough to do that.

DO NOT QUIT. Tell your employer you need to take 'sick time' due to being high risk.

Quitting weakens your claim, even if for a legitimate reason. Any loss of pay because your employer can't provide a safe work environment likely qualifies for at least partial unemployment.

I'm afraid the answer is no.  In order to draw unemployment in most states, one must be willing and able to work.  Some states, including NJ, NY, and CA, do pay benefits to the incapacitated as part of their unemployment system.  PA, however, does not.

Not unemployment. Doesn't matter what your reason is, (Unless its abusive -- physical or mental), you voluntarily left your job. But like squid said, you could collect social security. You're over 65, so you would qualify as long as you paid enough into it.

If you quit then you don’t get unemployment. You might qualify for disability and social security. 

Not unemployment, but you should be eligible for social security and medicare.  https://www.ssa.gov/benefits/retirement/

Probably yes if you live in the USA.

Maybe, maybe not.