Dear Leela, From my knowledge of Microbiology (Book reference:- Principles of Microbiology by Ananthanarayanan) only bateria stain gram negative or gram positive after preparing the smear using grams stain. Viruses are generally visible only under the electron microscope and not visible using the regular microscope. However, when viruses get attached to bacteria (bacteriophages), the bacteria that is attached to the virus get stained as gram positive or gram negative by virtue of its own (bacteria's) property and the virus does not get stained. Regards,