Feline leukemia virus (FeLV) is a retrovirus, a virus in which genetic information is contained in RNA instead of DNA. All retroviruses, including feline immunodeficiency virus (FIV) and human immunodeficiency virus (HIV), produce an enzyme called reverse transcriptase. Reverse transcriptase permits them to insert copies of their own genetic material into that of the cells they have infected. Although often mistaken as being the same virus, FeLV and FIV differ in many ways. One way they differ is their shape: FeLV is more circular while FIV is elongated. The two viruses are also quite different genetically, and their protein coats differ in size and composition. Although many of the diseases caused by FeLV and FIV are similar, the specific ways in which they are caused differs.

Cats infected with FeLV serve as sources of infection. The virus is released from the body in saliva and nasal secretions. It is also transmitted, though not as often, in urine, feces, and milk from infected cats. Cat-to-cat transfer of the virus may occur from a bite wound, during mutual grooming, and sometimes even through the shared use of litter boxes and feeding dishes. Transmission can also take place from an infected mother cat to her kittens, either before they are born or while they are nursing. FeLV doesn't survive long outside a cat's body, probably less than a few hours under normal household conditions.

There are many possible outcomes as to how successfully the cat’s immune system will react to the virus. About forty percent of cats extinguish the virus. Sixteen percent of these fight it off due to minimal exposure to it. The other twenty-four percent resist the virus at phase four, which will be described later. All of this usually occurs between sixteen to eighteen weeks after the FeLV infection begins. About twenty percent are able to put the virus into a latent stage, in which the virus will remain until the cat becomes stressed causing the FeLV to re-emerge. About five to ten percent of cats go through a sequestered stage in which viremia is limited, intermittened, or absent altogether. Approximately thirty percent of cats go through the disease from start to finish, normally resulting in death.

Once the virus has entered the cat, there are six phases to a FeLV infection. Phase one is when the virus enters the cat, usually through the pharynx where it infects the epithelial cells and infects the tonsillar B-lymphocytes and macrophages. These white blood cells then filter down to the lymph nodes and begin to replicate. In phase two, the virus enters the blood stream and begins to distribute throughout the body. Phase three starts when the lymphoid system (produces antibodies to attack infected and cancerous cells) becomes infected with further distribution throughout the body. Phase four is the main point in the infection, where the virus can take over the body's immune system cause viremia. During this phase the hemolymphatic system and intestines become infected. If the cat's immune system does not fight off the virus, then it goes onto phase five where the bone marrow becomes infected. At this point, the virus will stay with the cat for the rest of its life. In this phase, the virus replicates and is released four to seven days later in infected neutrophils (white blood cells), and sometimes lymphocytes, monocytes (white blood cell formed in the bone marrow), and eosinophils (another white blood cell). At this stage the cat's body is overwhelmed by infection and mucosal and glandular epithelial cells (tissue that forms a thin protective layer on exposed bodily surfaces and forms the lining of internal cavities, ducts, and organs) become infected. The virus replicates in epithelial tissues including salivary glands, oropharynx, stomach, esophagus, intestines, trachae, nasopharynx, renal tubules, bladder, pancreas, alveolar ducts, and sebaceous ducts from the muzzle.

The replication process of the FeLV virus is fairly straight forward. First the virus attaches to receptors on surface of the target cell. Then the virus penetrates the cell membrane, sheds its envelope and capsid, and releases its single stranded RNA and reverse transcriptase. Reverse transcriptase, with the help of the cat's cellular enzymes, makes the ssRNA into double stranded DNA. The dsDNA circulates and integrates into the nucleus and attempts to incorporate into the cat's chromosomes. If the viral DNA is successful in incorporating into the cat's DNA it transcribes to mRNA. The new viral RNA, proteins, and reverse transcriptase are synthesized, assembled, and then bud from the plasma membrane as a new FeLV virus to infect another cell. When the dsDNA is integrated into the nucleus it can have four possible effects: productive infection, latent infection, insertional mutagenesis (in which the viral DNA is incorporated into a critical part of the DNA and mutates the cell), and finally failure of integration (where viral DNA accumulates in the cell and is toxic to it).