Viruses are very small organisms that can cause diseases like colds and flu.
HIV is so small that 5,000 HIV's would fit side by side on the diameter of the period at the end of this sentence. HIV attacks the immune system, weakening its ability to protect you from certain diseases that healthy people don't usually get. This is why you don't get diseases like chicken pox more than once. But HIV tricks these guardians of health by implanting its genetic material inside them, crippling them even while multiplying itself safely within them. Some viruses cause disease quickly.