If a bug bite becomes infected and the infection is left untreated, the infection can eventually spread throughout the rest of the body, leading to sepsis in severe cases. The bacteria that is left in the bite that is causing the infection causes an infection in the skin called cellulitis, which causes increased pain, swelling, and redness in the area around the site of the bite. If cellulitis is left untreated, it can spread to the joints, bones, blood, and heart, becoming life-threatening if the infection continues to persist. If caught before spreading, a doctor can easily treat the infection through a course of antibiotics.