Saiga and Suitengu both contracted the virus when the Research Lab that Suitengu's team was sent to infiltrate blew up.
Since almost all the other Euphorics wouldn't have had anything to do with the infiltration mission and Suitengu mentions in episode 2 that the Piano Teacher would get more arms because of supposed deaths from people trying to become Euphorics, indicating that not everyone who came in contact with Kugura had the virus, I am wondering: how did the other Euphorics get the virus?
As you stated they both contracted the disease when the research lab exploded this got covered in Episode 10 "Suitengu Cometh"
Later on in episode 17
Suitengu listens to the melody of his music box and utters the name "Yui." He begins to reminisce about his childhood with his sister, Yui. On that fateful day, he discovers his parents hung and a man arrives to collect the debt they owe. Yui attempts to pay with money from a child's game, but the man discards the money and takes her captive and sends Suitengu off to army training. Suitengu always keeps his music box and survives the battles for seven years. On his last mission, he is sent to attack a research laboratory, but when the rest of his squad is defeated, he ends up becoming a test subject. He is healed and given an Euphoric power which allows him to defeat the army when they attack again. After arriving in Japan, he saves the lives of Tsujido, Makabe, and Niihari, and they become his followers. Then, he entices Shinsen into his plan for dominance and sets his plan into motion.
With that knowledge there are several possibility that could lead to more power users. The 2 most likely would be
