that's not necessarily the case..it's not uncommon for pet stores to sell "feeder" fish that are barely alive, already stressed from transport and underfed (to control ammonia and nitrites)
if they are not exhibiting any signs of disease they should be ok..
any fish, when stressed, becomes more susceptible to infections and/or disease, many are almost always in the water (after having livestock in it for a while) but are no problem for healthy fish