the infection is caused by the human immunodeficiency virus (hiv). after hiv is in the body it attacks and destroys cd4+ cells which are the part of the body's immune system that fights infection and disease. when hiv weakens or destroys the immune cells it may lead to certain illnesses or diseases such as some types of pneumonia or cancer that are more likely to develop in someone who has a weakened immune system. these conditions are a sign that hiv has progressed to aids. hiv is spread when blood semen or vaginal fluids from an infected person enter another person's body usually through sexual contact from sharing needles when injecting drugs or from mother to baby during birth. hiv is rarely spread by blood transfusions or organ transplants in the united states because of improved screening procedures. webmd medical reference from healthwise this information is not intended to replace the advice of a doctor. healthwise disclaims any liability for the decisions you make based on this information. © 1995-2011 healthwise incorporated. healthwise healthwise for every health decision and the healthwise logo are trademarks of healthwise incorporated.
what causes human immunodeficiency virus hiv infection?