Question

why do people who have HIV are at a higher risk from the flu or pneumonia than people who are HIV-negative?

Answer 1

HIV stands for Human Immunodeficiency Virus

The key here is "immunodeficiency", which means a deficiency in the immune system.

The immune system is responsible for protecting your body from pathogens, so when an organism's immune system is not working properly there are less defence mechanisms to act against pathogens.

Read this https://www.aids.gov/hiv-aids-basics/just-diagnosed-with-hiv-aids/hiv-in-your-body/immune-system-101/