3 of the 4 cats we have (siblings) were just dumped out on our street one day. They were only about 4 months old (thought we had 3 girls, but two morphed into boys a few months later LOL).
We also live next to what was once a chicken restaurant and they had a lot of cats ... all of them pets really. Over the last 2 years, they have managed to whittle down from about 30 cats to roughly 8. However, they are all sick in one way or another. And the owner feeds them the cheapest "cardboard" cat food, mixed with wet food. He mixes the food in a large white bucket and he's allowed the bucket and spoon to get moldy and icky. He also provides a water pan, but just adds water, rarely does his clean the pan out.
I often sneak over at night and clean the water pan and give fresh cool water. I also feed the cats normal good food. My heart breaks at seeing sunken in hips and runny noses. When I do a good run of feeding them, I notice a marked improvement in their health and weight.