Methicillin-resistant Staphylococcus Aureus (MRSA) is a type of bacteria that are resistant to certain antibiotics. When bacteria are resistant, it means that they can't be killed by common antibiotics. Many people have bacteria that don't cause any harm while they are on the skin outside the body. However, sometimes these bacteria get inside the body through a break in the skin and cause an infection. Infections caused by resistant bacteria, like MRSA, are more difficult to treat. They can also be very serious, especially if they are not treated properly or happen in deeper areas of the body like the lungs.