HIV reproduces itself very quickly, making billions of new viruses every day. Because the virus often makes mistakes when copying itself, each new generation differs slightly from the one before. These tiny differences are called mutations.

Some mutations occur in the parts of HIV which are targeted by anti-HIV drugs. This can result in strains of HIV that  are less easily controlled by the drugs. These HIV strains are called drug-resistant.

Drug resistant HIV strains vary – some may be highly resistant to anti-HIV drugs while others may be less so. When an anti-HIV drug is started, HIV that is fully susceptible to that drug disappears rapidly, leaving behind drug resistant viruses. These viruses continue to reproduce themselves despite the drug's presence. Diagram 1 on page 2 shows how this works.

Resistance is an important reason why anti-HIV treatment can fail. Viral load, which should drop when you start a new drug combination, may rebound if drug resistant HIV is able to emerge.

How resistance develops