Sometimes a shy fish can be intimidated just by being added to a tank with aggressive tankmates. One of the reasons I quarantine apart from disease issues is to give a new fish time to put on weight, gain strength get used to the routine of being in captivity, and time to recover from the trauma of being shipped half way across the planet.
As far as disease it would be better if someone else chimed in as I am no expert but white spots, eye cloudiness, breathing hard, and scraping on substrate, and not eating would be some signs.
Myles