The purpose of this study is to determine how laboratory tests called genotyping and phenotyping assess the effectiveness of antiretroviral drugs used to treat HIV infection. Some HIV-infected patients have measurable levels of virus in their blood even though they are taking combination drug therapy-including didanosine, stavudine, or efavirenz-against HIV. Genotyping and phenotyping can be used to test for resistance to a specific drug, thereby providing information that can help doctors decide on optimal drug treatment for a given patient. Genotyping identifies mutations, or changes, the virus undergoes that allow it to continue to grow despite drug treatment. Phenotyping involves growing the virus in test tubes with different amounts of drug and then calculating how much drug is required to stop its growth.