What Causes Recurring Pus-filled Bumps On Back?

My husband has had a bump on his back for 10 years now. His mom squeezed it one time and a lot of white foul smelling stuff came out. About a year ago another one appeared. Last night I squeezed both of them and a lot of white smelly stuff came out. Before last night you could see the bumps through his shirt they stuck out so much, but I seemed to have emptied them enough that they are almost flat now. Will these come back again and what are they? TAB