When determining the TCID50 for HIV-1 or HIV-2 (several strains) I have consistently found that the "virus alone" well, no cells, that I thought should serve as a negative control, has positive (even high) RT and p24 values (RTa ssay/ELISA) after 7 days. Should the virus be active, alone, for 7 days at 37ºC, and even despite media changes?