There are many species of bacteria that can cause infection, including Pseudomonas and Staphylococcus. Fortunately, your skin provides a protective barrier against many infections, preventing the bacteria from getting into your body.

However, surgical procedures involve opening your skin with an incision. This can allow the bacteria in and lead to surgical site infections. Sometimes these occur because the doctor did not scrub adequately prior to certain or properly sterilize an instrument. According to Johns Hopkins Medicine, there are three different types of surgical site infections.

Deep incisional infections

These occur underneath the surgical incision site. They extend beyond the level of the skin down into the muscles and other tissues underneath. An infection of this type can cause the incision to reopen on its own and produce pus.

Superficial incisional infections

These too occur at the site of incision but do not extend any deeper than the area of skin around it. You may observe pus from the wound site as well as other signs of infection, such as swelling, redness or warmth to the touch.

Organ/space infections

Bacteria in the spaces between the organs can cause infection. Surgical infections can also affect any internal organs involved in the surgery. Inflammation from the infection can also produce pus. Because it occurs deep within the body, the pus can collect to form an abscess, which also involves tissue disintegration.

Sometimes during surgery, doctors place a drain underneath the skin to allow fluids to escape. They typically monitor the contents of the drainage to evaluate how the wound is healing. Pus from the drain is an indication of infection of an organ or internal cavity.