Mersa (originally MRSA, abbreviation for Methicillin-resistant Staphylococcus aureus) was a bacterial infection primarily contracted by Sapiens. Most primitive antibiotics invented during the World War Era proved largely ineffective against it. However, a medieval remedy rediscovered late in the World War Era showed some ability to suppress the infection.
Adult Gelt proved to be largely immune to the infection as their bodies produced antibiotics naturally. This ability did not develop until the age of three or four. Also, particularly intense infections, such as the flesh-dissolving infection used in High Borns’ suicide capsules, could overwhelm this form of immunity.