What are you trying to understand? Are you asking whether Diagnosis A or B is better in predicting the presence of the virus?
The best way to do this is to stick it into a statistical package and generate a model. It takes five minutes and the answer is there is not a statistical difference between diagnoses A and B to a 95% confidence interval. If you want to do it manually, you should seek medical help. Ok, that was a bad pun. You can create a model that looks something like:
Virus = m * Diagnosis + b
Where Virus = 1 or 0 for yes or no, diagnosis = 1 or 0 for A or B and m and b are your standard variables for fitting a line. You can fit the line in Excel or by hand to find the slope and intercept, but what is important is the F statistic on the slope. That will show you significance. The question you need to answer is "Is this line any different than just a flat line" because a flat line means no significance at all. You can then use an online app
or a statistics table to see whether that value is significant. In this case, it is not.