Human immunodeficiency virus (HIV), which if left untreated can become AIDS, is a sexually transmitted disease (STD). according to the Mayo Clinic  it can also be passed through “infected blood, or from parent to child during pregnancy, childbirth or brest-feeding”. They also say that HIV “interferes with your body’s ability to fight the organisms that fight disease”. In other words it hurts your ability to fend off invasive germs in your body.