Feline immunodeficiency virus (FIV) is a Lentivirus that affects cats worldwide, with 2.5% to 4.4% of felines being infected. The p24 protein (also known as CA, or core antigen), is the capsid protein of the feline immunodeficiency virus. This is the most abundant viral protein, since each virus contains about 1,500 to 3,000 p24 molecules. This antigen is used for the early diagnosis of FIV infection. Levels of p24 increases significantly approximately one to three weeks after infection. It is during this time frame, before FIV antibody are produced, when the p24 test is useful in helping to diagnose infection. At the early stage of sero-conversion, antibodies against p24 antigen are the first to appear.