This paper provides an economic explanation for the non-existence of a vaccine against AIDS. It comments on previously claimed economic reasons why private laboratories do not have incentives to invest in an AIDS vaccine and provides a new one: private companies already operate in the market for treatment of already infected patients, which market is threatened by the eventual emergence of a vaccine that cuts the cycle of infection. Finally, the paper discusses some mechanisms to provide incentives for further private research in diseases where a treatment product already exists.