Subgroup A is the only subgroup which is transmitted between cats.

Vertical transmission from mother to kittens by milk or saliva or placenta.

Horizontal transmission by saliva during mutual grooming. Chance of recovery depends on age and maternal antibody.

FeLV is most common in the breeds which have been intensively bred, e.g. Siamese and Burmese, this is from crowding not hereditary/inbreeding.