OMG Sue! I went thru exactly what you are describing 2 years ago. At the time my household numbered 13 and a 9 month old kitten (Hendrix) tested positive for it. He was born feral and showed up on my property when a young kitten with his brother. Both were sick, both were vetted and supposedly tested while at the vet. We found out later the FeLV test was not run - not that it would have mattered as sometimes it just doesn't show up at a young age.
You are exactly right - online information is somewhat contradictory at best. I read all I could and also contacted a cat specialist that works with universities to keep up with the disease. Here's some of the conclusions I came to.
Hendrix's littermate Morrison also had the disease. In the 7 months that they lived in the house, they bonded EXTREMELY closely to another 3 year old brother pair Ruby and Pinky. The 4 of them spent about 50% of their time together, sleeping, grooming, eating, etc. Being young playful kittens, they engaged anyone in the house that would play with them, and the balance of the day was spent either with each other (25%), or mingling with other cats (25%). All typical cats living together in a household stuff.
When we found out that Hendrix was positive, we had every cat in the house tested. His brother was positive and Ruby was positive. All of the remaining 10 cats were negative, including Pinky. We had the entire household tested twice 30 days apart. Anytime a cat gets sick and we have blood drawn, we rerun the test as a safety measure (the incident has scared us for life).
My entire household is vaccinated regularly, including the FeLV vaccine. We free feed so cats share the same food and water bowls. We isolate cats upon entry to the house until they are tested or if they are sick, recover. Obviously all litter boxes are shared and we do the typical cleaning routine with them.
So what was different about Ruby, particularly over Pinky who had equal time with the kittens? Pinky/Ruby were also feral born littermates brought in at a young age. Genetically appeared identical - it was hard to tell them apart at times. Both were huge long haired red tabbies, apparently very healthy, in the prime of their life. The only difference was that Ruby was traumatized as a kitten - we found him entangled in a clematis vine at 8 weeks old in the heat of summer. His leg was injured and it took him a year to walk right due to restricted blood flow to his leg. Could this be the difference or was it just bad luck? My vet scratched his head on that one also.
Based on what I read online and got from the specialist, about 25%-33% of cats exposed to the virus actually take on the virus but spend the first 30 days or so trying to fight it off. They will test positive on the Elisa test at that time, which tests for exposure to the disease. A certain percentage of these cats, particularly those that are in the prime of life (as Ruby was) will fight it off. If they are vaccinated, they have a higher chance of fighting it off. I remember doing the math on the statistics and discovered that in this situation, about 10% of vaccinated cats with regular sustained exposure will succumb to the full blown disease (e.g. the virus goes into their bloodstream and is not reversible). The IFA test will test for the virus in the bloodstream and is considered the definitive test. So of the 11 cats in the house outside of Hendrix/Morrison, only 1 got it - fit the 10% profile exactly.
A cat with FeLV can live a long life but needs special treatments - the right food, medical supplements, and low stress. The big issue with a multi cat household for positive cats is that by nature of having so many cats, it is nearly impossible to keep it entirely stress free. You can isolate the cat, which we did with Ruby, but in his case, he was such a cat social cat that the isolation stressed him to an extreme. Our visits to his room simply were not enough for him. He quickly became very sick at being isolated but I would not risk further exposure to the rest of my household (Morrison and Hendrix both succumbed quickly to it as they were young and obviously were born with it).
I found a haven for FeLV cats that would have taken him in, but they helped educate me on the stress factor with FeLV cats - sometimes just moving them from their familiar environment is enough to put them over the edge.
I don't envy anyone going thru FeLV, and it is that much harder in a large household. If you need to talk, even if to vent, e-mail me and I'll pass along my phone number (PM's are down). I so totally emphasize with you right now.
From my household to yours, a big