An infection is a disease that can be spread between people, and an incision is a deep cut of the skin, coming from his knife in this case. An untreated incision can become infected, which in turn, can lead to other people becoming infected if that infected person’s blood comes into contact with an open wound of another person.

Also, note the wordplay between ill and infections. Since an infection is a type of illness, but ill is used to mean dope or awesome in this context.