A human body is made up of billions and billions of cells, but germs have only one cell. There are four different types of germs, or “microbes”: bacteria, fungi, protozoa, and viruses.
Bacteria (the plural of bacterium), fungi (the plural of fungus), and protozoa (the plural of protozoan) are all single-celled creatures but each type behaves in different ways. Like all living things, these organisms breathe, eat, shit, and reproduce. Not all germs cause disease; in fact, some of them help keep us healthy. Viruses are very different from the other three kinds of germs. Viruses do not breathe, eat, or shit, and they can’t reproduce on their own. Viruses are simple structures that straddle the line between living and non-living matter. A virus is made up of genetic material called DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), wrapped in a coat of protein. In order to reproduce, a virus must enter a living cell and take over some of that cell’s parts. Only then can the virus make copies of itself. HIV is a type of virus called a retrovirus. It is made up of two strands of the genetic material called RNA wrapped in a protein coat. This protein coat has spikes of something called gp120 all over it. These gp120 spikes work like keys. All of our body’s cells have receptors on them. These receptors act like locks to let some things in and keep others out. HIV, with its gp120 spike, or key, is able to enter cells that have what’s called a CD4 receptor, or lock. There are several stages in the reproductive cycle of HIV. At each stage, chemical messengers called enzymes help the virus make copies of itself. As HIV enters a T4 (CD4+) cell, the virus sheds its protein coat. Then its genetic material, called RNA, must be changed to match the cell’s genetic material, which is called DNA. An enzyme called reverse transcriptase allows the viral RNA to become viral DNA. In the next stage another enzyme, called integrase, helps the new viral DNA join, or integrate with, the cell’s DNA. Once the viral DNA is joined with the cell’s DNA, the cell begins to reproduce the virus, making hundreds of copies of HIV. Another enzyme, called proteinase (or protease), helps put together the parts of the virus. When the new viruses are ready, they burst out of the cell and into your blood or lymph.
