Which of the following targets the immune system and weakens people's defenses against many infections and some types of cancer that people with healthy immune systems can normally fight off?

The disease that targets the immune system and weakens people's defenses against many infections and some types of cancer that people with healthy immune systems can normally fight off is HIV/AIDS.

Here's a step-by-step explanation:

1. HIV (Human Immunodeficiency Virus) is a virus that attacks the body's immune system. If HIV is not treated, it can lead to AIDS (Acquired Immunodeficiency Syndrome).

2. Once a person has HIV, the virus begins to multiply and to attack the immune system. Specifically, it targets CD4 cells (T cells), which are a type of white blood cell that plays a major role in protecting the body from disease.

3. As the virus continues to multiply, it damages and destroys these T cells, weakening the immune system. This means the body is less able to fight off infections and diseases.

4. When the immune system becomes severely damaged and the number of CD4 cells falls below a certain level, a person is diagnosed with AIDS.

5. People with AIDS are at high risk of developing serious illnesses that the body would normally be able to fight off. These include opportunistic infections and cancers, such as Kaposi's sarcoma and certain types of lymphoma.