There's an outdoor cat who comes to our home very often. I don't know what has happened but her fur around neck has almost gone. Even on her head and ears, the fur has started to fall out. Also she has little blood oozing out of the areas where her fur is thinning out. Is she suffering from some kind of disease? What can I do to help her out? Note that she's a feral cat and doesn't come into our hand for us to take her to vet.
Have you ever heard of Trap-Neuter-Return? (in your case, it would be Trap-Heal-Return). It's a method of catching a feral cat in order to either neuter it or getting it the medical care it needs, then releasing it back into the wild. If you can invest in it, I recommend using this method to take it to a vet. Some local animal organizations can also do it for you, if you have any nearby. Here's a good video on how to: https://www.youtube.com/watch?v=wF_omFE7Etc
When choosing a vet, please be sure it's a vet comfortable with taking care of feral cats.
I would have to see a photograph, but my best guess would be a bacterial infection, which is common in feral cats, causing inflammation and hair loss. This is only a guess, though, so your best bet is to use the method described above and take the cat to a vet!