Hello As you know , your self that there is an abscess in the arm pit ( under arm ) ,yellow pus is also oozing . Such type of abscess in the armpit may be due to these reasons, these include: 1 Staphylococcus infection , as this is most common cause of abscess formation in the arm pit . 2 Tubercular lymphadenitis is the another reason of such abscess . 3 Due to secondary infection , as some times primary infection is on elsewhere . You didn t mention , which type of treatment you are taking . When such abscess occurs I prescribe broad spectrum antibiotics and anti inflammatory drugs for 10-15 days . 2nd option is consult a surgeon and he will deal . Because sometimes such large abscess require surgery , culture of pus and drug sensitivity test . So at present the best option is consult a surgeon and get his opinion. Good luck.