What happens with acne is hair follicles get blocked with oil and dead skin cells. Acne filled with pus or blood means that it is infected. When it comes to treating them, using topical benzoyl peroxide is quite helpful while for severe cases, oral antibiotics such as doxycycline may be necessary. Make sure you continue your treatment regularly and keep your face clean so as not to get more pimples. You can also visit a dermatologist for further treatment.