Pneumocystis is an organism normally found in
the human lungs that can cause pneumonia. it
seldom causes problems in individuals with
healthy immune systems. however, people with
aids sometimes become seriously ill with
pneumonia. this is most likely due to the fact
that individuals with aids have
(1) inherited a tendency to contract pneumonia
(2) difficulty fighting off infections
(3) an allergy to this organism
(4) hormones that strengthen the infection

The correct answer is (2).

HIV that causes AIDS  infects the cells of the immune system and in particular the T-helper lymphocytes, or CD4 T-cells, which are crucial to the immune system and immune response . The virus kills these cells until their number goes down gradually. It may take a few years before the decline gets to a level that the immune system is too weak to fight off infections.

The lower the count  of CD4 cells becomes, the more susceptible a person becomes to viruses and infections that an otherwise healthy body would fight.