Why HIV patients have greater virulence of M.tuberculosis?
If you read this article, you will find that CD4+ and CD8+ T-cells are probably the major mediators of the immune response against M. tuberculosis. Since HIV severely depletes the number of CD4+ T-cells (and to a lesser extent other kinds of lymphocytes), it stands to reason that the frequency of infection and virulence will be substantially increased in such patients.