Maggots can be harvested from rotting carcasses.
You can get an abrasion by falling or go into piranha infested water. Wait for the wound color to change from red fresh blood to green infection, it shouldnt take long and youll have a fever pop up. Select the maggots from your backpack and apply to the wound. The wound will have a circle icon that will change from white to green if the wound is appropriate for the treatment and ready to have the maggots added.