Which type of infection follows an untreated gunshot wound?

Think toilet !: If you understand that a shell is not a clean item and add the contents of the barrel and what it has traveled through and you basically have injected a dirty toilet into the tissues. These are very tricky wounds as the energy in the shot also causes damage that takes a while to show itself (days). Better to be seen by a doctor rather than risk a horrific infection and loss of function.

Velocity? Organs?: As dr. Miller points out the bullet or gun may not be clean and the presence of bacteria may depend on the velocity and energy of the bullet. However penetrating injuries to the bowel can cause devastating fecal soilage of wounds with fatal consequences. Other concerns include tetanus status. Your risk of infection is also related to your overall health and vulnerability and immune state.