The body's own defense system is a little like a police force. Many of the body's cells work together and each has a different function. Some sound the alarm when viruses, bacteria or fungi (pathogens) enter the body. Others - so-called killer cells - can destroy the pathogens that cause illness.
To ensure that all of the cells of the body's police force work together smoothly, some cells work as managers of the operation. This is the job of the so-called helper T cells (also called CD4 cells). They give the orders to the ‘special units’ of the immune system - they tell the other cells what to do.
HIV attacks these helper T cells, and thus puts the command center of the immune system out of action. When pathogens enter the body, the immune system can no longer restore order. In the worst case, the body is no longer able to defend itself from disease-causing pathogens.
Luckily things no longer have to get that bad. Medications can prevent the replication of HIV in the body - and thus protect the helper T cells.