A substance obtainable from Staphylococcus Aureus bacteria or from a Staphylococcus Aureus bacteria culture, the substance characterized in having at least one of the following characteristics: (a) stimulates proliferation of bovine T-lymphocytes in vitro; (b) accelerates healing of skin wounds in mice and pigs; (c) prevents AIDS-like disease (ALD) in mice infected with ALD virus (ALD-V) when injected substantially simultaneously with the virus; and (d) reduces the viral load in Rhesus Macaques infected with the hybrid simian/human immunodeficiency virus (SHIV) when injected into chronically infected Macaques, or a derivative or fraction of said substance which retains at least one of the characteristics of the substance. Also disclosed is a novel Staphylococcus Aureus bacteria, and a pharmaceutical composition comprising as active ingredient the substance and a pharmaceutically acceptable excipient.