1. HIV is transmitted through sexual contact, contaminated needles, blood transfusions, and from mother to child during pregnancy or birth. It attacks CD4 T cells in the immune system. 2. AIDS develops when HIV has destroyed enough T cells to weaken the immune system, making a person vulnerable to opportunistic infections. 3. Some diseases like allergies, cancer, diabetes, asthma, and heart disease are not spread between people but have other causes like genetics or environment.