AIDS, stands for Acquired Immune Deficiency Syndrome, is a deadly disease. It malfunctions the human body's defence system, making the infected person extremely vulnerable to diseases, and eventually cause death.
AIDS can be spread in many ways, but the main medium is through having unprotected intercourse with an infected person. Apart from that, AIDS can spread from mother to new born baby, or sharing of needles, shaving blades, or any means of blood contact.
Once infected, it may take years for the person to notice, and in these years, the infected person may not know and can infect many more, who in turn go and infect others, just like an exponential growth.