The only way a virus can reproduce is to infect a host cell, hijack the cell's machinery to make copies of itself, and then move on to infect other cells. But not all viruses do things the same way, and these differences can have consequences for everything from viral evolution to treatment.
Different viruses use different nucleic acids (DNA or RNA) as the genetic material. Some, like herpes, use DNA. Some, like the flu, use RNA. And some (like HIV) use both, alternating back and forth. This third type of virus is called a retrovirus. Retroviruses use the enzyme reverse-transcriptase to make RNA from DNA (refer to Figure 18-1).
Different viruses have different mutation rates, and viruses that use RNA (either all or some of the time) can have very high mutation rates. HIV is one such virus. Its reverse transcriptase makes a lot of mistakes, leading to many slightly different viral types.
Some viruses replicate in the host cell and then the progeny go on to find another cell to infect. But others, including HIV, will also sometimes slip their DNA into the host chromosome. When the HIV genome is present as DNA, it can integrate into the host chromosome and hide there. It's just a string of A, C, T, and G bases and the immune system can't find it. Neither can current antiviral drugs. Hopefully, drugs that can find the virus can be developed. HIV researchers know what sequence to target; it's just a matter of figuring out how.
What Is HIV?
HIV stands for human immunodeficiency virus. HIV attacks and impairs the cells in your immune system. HIV itself doesn't kill you; you die of any of the infections that your body usually would be able to fight off.
Several HIV viruses exist, and they can be divided into two major groups: HIV-1 and HIV-2. Both groups of viruses behave in a similar fashion, but the global HIV epidemic is primarily the result of HIV-1, and HIV-2 is confined mostly to one region of Africa.
The reason I say groups or types of viruses rather than species of virus is that scientists really have no idea what constitutes a viral species. Head to Chapter 8 for a discussion of what defines a species and why categorizing viral or bacterial organisms is so difficult.
Understanding the replication process that HIV uses is key to understanding why it's such a hard-to-treat disease. As the preceding section explains, HIV is a retrovirus; as such, it copies its RNA genome to DNA and then back to RNA. This replication process is important for two reasons, as explained in the following sections.
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