Feline infectious peritonitis (FIP)=Spreads with inhaling or ingesting the virus in saliva & feces; either by direct contact with infected cat or virus contaminated surfaces such as bed, toys, bowls.
Feline immunodeficiency virus (FIV)= Primary route for tranmission is cat-cat bites. Transmission by sexual contact, in utero or by milk from the mother, although possible in theory, is very rare.
Feline leukemia virus (FLV)=Can spread by nose-nose contact, bites, mutual grooming, shared bowls, blood transfusions.
Panleukopenia (distemper)=Spreads by feces & urine of the infected cat & from mother to kitten in utero.
As being more social creatures than our indoor cats, how many of you have tetanus, tuberculosis, hepatitis B, meningitis etc. shots regularly ?? So why do strictly indoor cats would need all of these shots ? Yes, the vets are trying to get you all the shots they have. For exp. my vet was about to give the new giardia shot to our both cats. What has giardia got any business with my strictly indoor cats who drink the same water as we do (we even filter the tap water with purr) ?? She also offered the heartworm vaccine !!?? They always want to scare you by telling what would happen if the cat somehow caught the disease ! Yes although small, possibility is a possibility. But vaccines contain adjuvants, preservatives such as thimerosal (mercury), aluminium etc. which are very dangerous in high doses. These chemicals may accumulate in the body & can cause damages in kidneys, nervous system and immune system. So this second fact seems more possible to me than the 1'st one... That's what I think
By the way, FLV & rabies shots pose the danger of "vaccine induced sarcomas", which is I think what you are talking about. Although the possibility is relatively small, it increases when the shots are done at the same time & at the same location.