The short answer The expression, $p(a|b)$, read as the probability of “$a$ given $b$”, or as the conditional probability of event $a$ with respect to event $b$, cannot generally provide the probability of “$a$ given $b$” using the same probability function that assigns probabilities to $a$ and $b$. This is because such an event does not exist in the same space as $a$ and $b$. In fact, $p(a|b)$ is just a notation expressing the ratio of two probabilities.

Dear colleagues/students/visitors We need, more than before, serious and ethical research. We need to take volunteer, collaborative and conscious actions in order to fight against Covid-19. We are much more equipped than we were 100 years ago against the Spanish flu but we have to organize our assets in a smarter way: typing a complicated python code in less than 5 min and plotting an exponential curve are definitely not the skills that we need now.