The parasite can be fetal if it is left untreated. As some fish can have a natural resistance to it, it can be very hard to predict how fast it will progress.

One of the best precautions that I have learn (and had to learn the hard way) is to quarantine new fish for around 4 weeks before adding them to my tank. I will also give them a preventative treatment of prazi-pro for the things I may not see.

As for identifying a sickness before it becomes serious, you would need to look for any chances in behavior, appetite, or any minor change in appearance. Some or all of those signs can be a potential symptom indication a sickness or disease. Early identification can be a very difficult task as the best of times which is why I like to use quarantine tanks.