who is Thomas Bayes

P(A|B)=   P(B|A) *  P(A)    / P(B)
the theorem expresses how a degree of belief (I am Covid positive) 
  	expressed as a probability, 
    should rationally change to account for 
  	the availability of related evidence
where A|B are events and P(B) not 0 and P(B) not 0.

P(A|B) are P(A|B) is a conditional probabilities: 
 the likelihood of event A occurring is conditional on B is true.
 P(B|A)} is also a conditional probability: 
   the likelihood of event B occurring given A is true.
P(A) and P(B) are the probabilities of observing A and B respectively; 
   are known as marginal probabilities.
A and B must be different events.
https://www.britannica.com/biography/Thomas-Bayes