My suggestion is that the infection be transmitted via prolonged contact with the leotard. Like that one sleepymaid comic, which I will upload once the Booru mods get my permissions straightened out, I picture the infection cycle going something like this.
1. Haigure girl uses the dance to entrance a normal human. The human becomes engrossed with the haigure girl's crotch area as the leotard massages it.
2. Once fully hypnotized, the haigure girl orders the human to strip naked. He does so, while still resisting a little.
3. The haigure girl initiates some form of erotic contact with the human. Within a few minutes of being touched by her leotard, the human's body begins to secret a thick gel that congeals into a leotard of their very own. There might be additional changes as well, particularly in boob size, hair color and skin color. Some converted humans might tend towards a more ganguro appearance, and some might remain entirely unchanged.
4. At that point the brainwashing is complete, and the new haigure girl partakes in her very first haigure, with orgasmic pleasure as the leotard bites into her crotch. After feeling that amazing sensation, she is completely loyal to the devil king and thinks only of spreading the joy of haigure.