Follow up studies suggest that a potential mechanism for this is that pre-existing immunity to adenovirus serotype 5 (Ad5) from natural infections caused the Ad5-based HIV vaccine to stimulate memory responses to Ad5, expanding activated CD4+ T cells expressing CCR5 at mucosal sites; this inadvertently created a target-rich environment for HIV, increasing susceptibility to infection rather than preventing it [17]. The gene discussed is CCR5; the disease is infection.