That is a common occurrence. Stress allows fish to get sick, few common aquarium pathogens can infect healthy, unstressed fish. Stress them and something WILL cause an infection.
hobbyist quarantine is a joke that causes a lot more harm than good 95% of the time. Few people have proper QTs that don't cause even more stress during an already very stressful time. A proper quarantine takes months, that is AFTER the last signs of anything have been seen. To think that a few weeks in a cramped, undecorated, uncycled tank is a good thing should lead people to rethink their role in the hobby.
FYI garlic has been shown to actively kill parasites. Some people have used it to safely treat white spot (not ich in SW
) in reef tanks. The best form is freshly pressed garlic but some people have been successful using New Life Spectrum's Thera+A Formula, which has enough garlic to kill some parasites. It is one of very few (if any other) safe but effective methods to treat white spot in a reef setup.