Hi Bionca, Merca is basically MRSA which is Methicillin-Resistant Staphylococcus Aureus. It is a staff skin infection and is super infectious However
, I am betting if your sister in law was diagnosed with this then she was also prescribed the appropiate antibotics for it. She will always have MRSA but it will not be active if she has been treated. She will need to be very careful in the future though. Here is a link on it I found.