Fact 1: Immunodeficiency occurs when the immune system is not working properly.

Fact 2: HIV, the human immunodeficiency virus, is a retrovirus.

Given the two facts above, answer the question "what causes the immune system to not work properly?" with the following options: 
- HIV
 - lack
 - DDT
 - p53
 - antibiotics
 - h2o
 - memory cells
 - HPV


 

 HIV        