Human Immunodeficiency virus (HIV)

It is virus that attack the lymphocytes (B-Cells and T-cells)  of immune system which are responsible for the adaptive immune system.

• Mechanism of spreading: It binds to the DNA of cells by that: It contains RNA and Enzyme called Reverse transcriptase that converts RNA to DNA then this DNA binds to the DNA of cells and replicated with cells and also it is transcripted and translated with body enzymes producing more and more viruses.

• the hardest thing is that no drugs can stop it because it is RNA meaning it is changed all time making drugs inefficient. also it evolves if the medium around it is changed giving new virus(suc as if you take a drug to stop it this drug affect your body changing the internal medium then this virus change from it self like the evolution principle of darwin). so it is the worst virus because: 

1.  no drugs are effective 
2.  High ability to change according to the medium
3.  It is RNA not DNA meaning increasing exchange in its structure
4. It damage the most important system in the body ( immune system) making body with no defenses increasing infection with different other diseases