BAD OR SLOPPY RESCUE
A few humane groups do not follow the isolation guidelines and, therefore, pass diseases (including FeLV, distemper, mange & ringworm) on from litter to litter.
Isolation should be followed until the cats have been cleared of all health issues. This includes testing cats for FeLV and FIV (many vets recommend the kittens be 11-12 weeks of age for this test), stool samples, and waiting 7-10 days, the standard incubation for URI's and Distemper.
If a group boasts that it lets the litters of kittens run together and they have not been tested and incubated, beware! This is sloppy rescue! Passing diseases that kills kittens and cats is not the goal of cat rescue! It is easier to house many kittens together than to isolate litters. This is quantity rescue, not quality rescue.
Although the person(s) may have a big heart, they are doing quantity, not quality rescue.