I've yet to see a pet shop that can guarantee you healthy, disease-free fish. They all use a mass filtering system that circulates the water from every tank into one giant pool of questionable purity. They tend to constantly dose said pool with various medications to keep disease at a minimum. The result is that any sick fish will transmit it to every fish, at least potentially, and that every fish is always medicated, so you have a more fragile fish when you bring it home.
Some stores may avoid the constant medication, but many use this practice to help protect their stock at least a little. Of course, it has many drawbacks.
I've had a similar experience with a new fish causing deaths in my old fish. However, to use your rocket science quote, it doesn't take rocket science to know the only safe way to buy new fish is to use a quarantine tank.