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, environment, or lifestyle factors.