im not sure i dont use ro water for any of my shrimp. never have. i think its probably always around but stress on the shrimp is what causes them to get it. kinda like people and getting a cold. there are all kinds of microscopic things in a tank. what caused my rili to get it i think was A)new tank, they had only been in this tank a couple weeks; and B) the temps went up (due to some one playing with the thermostat on the house lol) so stress and higher temps.
on a side note temp place a big roll in all kinds of bacteria, that is why when cooking its a must to store food right or the bad bacteria make people sick cause it grows ie food poisoning