Garlic is a good start if this is a worm. More commonly a fish that eats and eats but remains thin will have a worm infestation. It's not all that uncommon. It can also be related to the types of foods used as well. Pay close attention to the feces output when possible. Check color, consistancey and any sign of little "string" bits mixed in. Gross but necessary.
Feed garlic daily and please post food types used/how often along with water parameters and general tank info.
Yes but how easily will depend on the species of fish. Worm infestations are more commonly transfered through consuming infested items. Tangs in particular being the number one candidate as are Bfly's. Tangs due to their habit of eating the feces of tank mates and Bfly's due to their dietary preference for worms in general. All fish are potentially at risk but is not a guarantee the problem will manifest.