The goal of new combination treatments is to get the amount of virus in the body as low as possible. Treatments belong to different classes of drugs, but they all work by stopping parts of the virus so the virus can't make more of itself. Research has shown that using combinations of drugs from different classes in order to stop different parts of the virus from working is a better treatment strategy than using only one or two drugs.

One class of drugs called nucleoside analogs stop a part of the virus called the reverse transcriptase enzyme. Drugs in this class are AZT, ddC, ddI, 3TC, d4T and abacavir (Ziagen). A second class of drugs called non-nucleoside reverse transcriptase inhibitors also work at stopping reverse transcriptase. Drugs in this class are nevirapine, delavirdine and efavirenz (Sustiva). A third class of drugs are protease inhibitors which stop the protease enzyme part of the virus. Drugs in this class are indinavir, saquinavir, ritonavir, nelfinavir and amprenavir (Agenerase). Researchers are working on new, possibly more effective drugs in these classes and others as well.

HIV does not always make perfect copies of itself. With billions of viruses being made every day, lots of small, random differences almost like mistakes can happen in any new virus that gets made. The differences are called mutations. Mutations that change the parts of the virus the drugs are meant to stop can keep the drugs from working. When a drug no longer works against HIV, this is called drug resistance. The virus with the mutatation is resistant to the drug.

For example: A person may have a few HIV viruses in their body that, by chance, have a mutation that stops AZT from working. AZT will still work against the HIV that hasn't mutated, but the mutated virus just keeps going, infecting more T4 cells and making more copies of itself. The new copies will be resistant to AZT.

Eventually, this AZT-resistant HIV will be the only HIV in the body, and there will be just as much virus as there was before the person started taking AZT. AZT will no longer work for that person. The same thing can happen with the other kinds of anti-HIV drugs.

What can help stop drug resistance? What's important to know is that drug resistance doesn't happen because HIV is clever and works out how to get around the drug. Resistance mutations happen randomly - with billions of viruses being made in the body every day, there's a good chance that at least one new HIV virus will be drug resistant.

This is why researchers think that strong anti-HIV drugs like protease inhibitors need to be taken on schedule, and at the right dose. Researchers also recommend combinations of anti-HIV drugs for the same reason. Keeping HIV production in the body at a minimum lessens the chance of a drug resistant HIV virus being made. And if a drug resistant HIV virus doesn't get made, the anti-HIV drugs can keep working much longer.