I have a quarantine/hospital system in place. I have a spare empty tank with a light, a spare heater, and a spare powerhead. In one of my tanks I have an air pump driven sponge filter bubbling away, and if I need to either quarantine a new arrival or hospitalise a sick/bullied fish I do a water change and drop the used water into the spare tank, put the sponge filter, heater, and powerhead, and there it is. People saying it's not feasible to have a QT tank just need to plan ahead a little.

Also, whitespot has been mentioned and other ways to prevent this disease from affecting tank stock. There are more diseases prevalent in the hobby than just whitespot, and ozone/UV are pretty much ineffective against such disease. If you get an outbreak of Velvet then all the looking at a fish before you buy it, and taking care not to stress it etc. will be no use to you.

Each to their own of course, but I sit in the camp of "Don't let disease into your tank" rather than "Let it in and fight it when it shows up".