If you added freshwater shrimp to your tank, then the water they were in might have been infected and contained a few parasites. These have since multiplied and caused the infection.Still no improvement today. The only thing I can think of is that added a few shrimps for the angels the other day and that they were carrying something?? Just a thought been racking my little brain to think of how it could have started.
If you added brineshrimp then they won't have introduced anything because they are kept in saltwater and although the parasite lives in fresh, brackish and saltwater, it cannot survive the change from fresh to salt or vice versa.
edited to add something about the no improvement statement
If it is whitespot then it will take a couple of days before the spots fall off the fish. You should start to notice a difference tomorrow or the next day. The fish should start to lose the spots. If you have medication in the tank then it will kill the parasites before they get a chance to re-infect the fish.