in disease progression, particularly for those who have a high prevalence of chronic and recurrent acute infections and poor access to medical care. Often, survival depends upon the initial AIDS-defining illness. Co-infection with DNA viruses such as HTLV-1, herpes simplex virus-2, varicella zoster virus and cytomegalovirus may enhance proviral DNA transcription and thus viral load as they may encode proteins that are able to trans-activate the expression of the HIV-1 pro-viral DNA. Frequent exposure to helminth infections, which are endemic in Africa, activates individual immune systems, thereby shifting the cytokine balance away from an initial Th1 cell response against viruses and bacteria which would occur in the uninfected person to a less protective T helper 0/2-type response. HIV-1 also promotes a Th1 to Th0 shift and replicates preferentially in Th2 and Th0 cells. This makes the host more susceptible to and less able to cope with infection with HIV-1, viruses and some types of bacteria. Ironically, exposure to dengue virus seems to slow HIV progression rates temporarily. See also CCR5 References External links "Gene Variation May Raise Risk of H.I.V., Study Finds" from The New York Times, 2008 HIV/AIDS