The present invention pertains to the prevention or lessening of disease in cats caused by Feline Immunodeficiency Virus (FIV). Prevention or lessening of disease is understood to mean the amelioration of any symptoms, including immune system disruptions, that result from FIV infection. The invention provides for a plasmid which encodes the FIV genome where said genome has had a portion of the gag gene, specifically the p10 (nucleocapsid) coding region, or a portion thereof, deleted. This deletion prevents the production of functional or whole p10 protein, which in turn, prevents the packaging of RNA into virions produced from transfection of this plasmid into an appropriate host cell, resulting in virions which do not contain RNA. Such virions will be described as “empty” virions. The invention also encompasses host cells transformed with the plasmid which produce the empty virions, and the empty virions themselves. In another embodiment, the invention encompasses vaccines that comprise one or more empty virions described above, with a pharmaceutically acceptable carrier or diluent and a pharmaceutically acceptable adjuvant. In yet another aspect, the invention provides methods for preventing or lessening disease caused by FIV, which is carried out by administering to a feline in need of such treatment the vaccines described above.