I set up a same day date. And then received a text from my son, whom I just stayed with for a couple of days. That he tested positive for Covid. So I let the provider know that we need to cancel. She wants a cancelation fee. I said no! I let her know ASAP. I have no symptoms.

If she has a cancellation policy, then that is that ... it doesn't matter that you gave her prior notice.  What matters is whether you gave her enough advance notice and respected her cancellation policy.

The only thing that's relevant is, were you aware of her cancellation policy and did you agree with it explicitly or implicitly? The fact this was a same-day date means it's highly unlikely she'll be able to fill the time.
So, yes, I'd say you owe her the fee.
Personally, I have tried to avoid girls with cancellation policies.

that changes or impacts the commitment you already made with her, then you owe her a cancellation fee if it was clear that was a condition of her accepting your appointment request.  If she did something that changed your understanding, like asked you to come at 3;00p instead the time you booked at 11:0, then you would not owe the fee if you cannot accept the change in the booking initiated by her.  It doesn't would like she did anything wrong, so I think you need to suck it up on this one.  You get "gentleman" points for not wanting to potentially expose her, but she can't pay her rent with those.  

If you are able to make the appointment, I would put the decision to cancel on her. Tell her the situation (you were staying with someone who just got diagnosed and you are asymptomatic), say that you could make the appointment but don't want to infect her, and she can make the determination if she wants to take the risk. I have done that when I have started coming down with a cold but am good enough for a date. Every provider I have done this with has taken the pass and appreciated me being honest about my condition.

You let her know as soon as you found out and you were being considerate since you had been exposed to Covid.  When she said she wanted a cancellation fee, you should have forgotten about being a nice guy.  You should have said: "OK, in that case, let's go ahead with the date.  I'll pays my money and you takes your chances."

Then tell her you're not showing symptoms and you'll be over at the appointed time. See if she cancels on you. I'm being sarcastic, but not really.

In order to save a small amount of money you think it's okay to let someone who might make a really bad call and then go about spreading the joy is smart?

I didn't see anywhere in his post where he mentions a deposit, only that she asked for a cancelation fee after he canceled.  

I looked again, she does not have a web site, I found her on P411,

If they said that they wanted to follow through with the appointment, I would probably still cancel as i would recognize that the provider is using questionable judgment. As i said, every time have had do this,  providers have opted to cancel.

The only understanding needed in this scenario is that Nalit owes the provider a cancellation fee. If Covid becomes an acceptable exception to that, we’ll see a spike in fake Covid cancellations.

How soon after the original call did you call to cancel? You said, "ASAP". How long was ASAP?  
.
I mean, I have made appointments to see someone in the real world, checked my calendar and had to call back to 2 - 10 minutes later to cancel or change the original appointment.  I mean: 10AM, call to arrange meeting at 4PM (maybe while I'm driving someplace or not with my calendar easily in hand); then check and, ooops, my 330 is for a full hour, not 30 minutes; 10:10AM, call back to apologize and change plans.  
.
What happened during those 10 minutes, calendar-wise? Probably "not much." And if it DID happen (the Provider in question turned down another offer), she can call back that guy and say, "I made a mistake. I AM available to see you at that time."  The 10 minute "error" might propagate, but not likely.  
.
Compare to: ... appointment person arrives at 4PM. Sorry, my 330 is going to 430; please wait here. And then I have to leave at 5PM for some other commitment and I don't think 30 minutes is enough time to cover everything so I suggest rescheduling. Now THAT would be disrespectful and rude and worthy of a cancellation fee.  (I don't treat people that way; never happened; just an example.)  
.
What was the delta t (gap between your appointment call and your cancel call)? 6 minutes or 6 hours?

P411 should have a section for the providers policies. If she doesn't have a web site and doesn't have a cancellation policy in that section and she did not bring that up in you scheduling then she only has a cancellation policy in her mind.

If she had a cancelation notice on her site, and you canceled inside of her allowed time frame...youre on the hook for it. Doesn't matter if you booked same day, if she has a 48 hour prior to date policy...youre on the hook. Same day appointments mean you dont have a grace period. Regardless...

You know these folks??

...about her cancellation policy...then pay it!!!!...its not her fault...we are talking life and death here....never promise to invest more than you can afford to lose....btw-no symptoms means ZERO w this virus...