HIV or AIDS is very different from the flu. First of all, HIV can be contracted through sexual contact or sometimes through the blood. There are some people who become HIV positive because their mothers are also afflicted with HIV or AIDS. This is something that they have to live with all their lives.
HIV will have a natural tendency to kill or damage your overall immune system, which means that you will be more susceptible to different diseases. At the same time, people can contract HIV from you. You can get something simple like flu, and it will become a full-blown case of pneumonia.
Flu is something that you can contract through a virus. It may be airborne, or you may get it from someone that you were in close contact with recently, but it is rare that it becomes serious.