Unfortunately this can happen. I am sorry for your losses. In this day and age, with both wild and captive bred fish, I am seeing more and more deaths of fish. I will always qt my fish. No exceptions.
Since you do not have a qt system and are rather new to the whole process, I recommend using pre-qt'd fish. There are a few out there.
As above, you added a nasty disease to the tank. If all fish die, leave tank fallow for 4 months and then either only do captive bred from captive only systems, or a qt'd fish.
Good luck. Even the much more experienced make mistakes. Just see my build. ;Happy