How To Treat Itchy Pus Filled Spots On Shaft Of Penis?

I am 40 and married for last 11 years having 2 healthy kids. I often have 3-4 small spots on the shaft of the penis. These appear after every 3-6 months. These spots are filled with bit of white fluid like puss. After 3-4 days get converted into small wounds, which get cured in 15-20 days time. It is bit painful to have sex during this period. Iching in these spots is there but low, it is normally odourless. Kindly guide why do these apprear. Also suggest medication and precautions.