Question

Over time, people with AIDS become very sick and are unable to fight off infection, why does this occur.

Answer 1

HIV is similar to other viruses, such as those that cause the "flu" or the common cold. But there is an important difference – over time, your immune system can clear most viruses out of your body. etc. Also sometimes your immune system isn't strong enough to fight  back.. 

Answer 2

People with AIDS lose their ability to fight off infections because AIDS weakens the immune system. (The immune system is what helps you to prevent illness and get better quicker.) AIDS itself also causes several health issues, like fatigue and difficulty swallowing. AIDS has so many symptoms it's hard to cure them all at once, and all this is happening while your immune system is weakening. Hope that clears things up :)