This is more of a Discovery than a Question. I Read & Tried all the Posts about Hairloss & Weightloss: Fleas. yes common, allegies yes also common.
SOLUTUION FINALLY: I use to have dry food for them all day & feed wet food at night. BUT: I started feeding my Cat Twice or more a day. Fresh food each time, not stale. Wet food morning & evening & dry food throughout the day. She is Now Full of hair & Healthy. I found my cat(s) love a fresh chicken egg once in a while (raw) & I read a Doctor say it's good for them & their coat.
ALSO: I Got away from the Most Expensive LEAN Diet & started giving them some Fat. Now: Less Kidney Problems, Better Coat & Less throwing up (although throwing up is normal for cats who groom) - Some Fat in a Diet is normal, natural & healthy.
Other Tips: Put yourself in 'their Shoes': a little variety, yet consistency in feeding times. Rinse well after baths (be gentle & don't panic during baths, don't get water on their head or face while bathing (too much at once))
Spend gentle time with them, let them hide in that place you may know where it's at, but let them feel they are hiding, never (if you can) get them or let them know you know where they are; but you do know they are safe. Let them have some natural time playing. They will love & trust You unconditionally, well worth it for you & them. :-)
Lastly: If a Cat or Animal is Mean: That means Abuse! Don't confront the suspected abuser, just take the cat out of or avoid the situation. Put up cameras. Do what's best for the innocent life.
If an Animal is not feeling well, they may appear grumpy or lethargic. Give them space or seek a Dr. - but sometimes we all just need to be alone once in a while too.
Smell your hands before petting, a cat or animal doesn't like to have dirty or smelly hands rubbing it into their fur (after handling meat or whatever - make sure your hands are clean)
Signed Cat & Animal Lover & full time care giver.