Demon Sacred (2003)

Blurb:
There is a sickness, named the Return Syndrome, that causes people to physically regress toward infancy until they simply disappear. If someone you loved became afflicted with it, would you give up? No! You’d do anything humanly possible to help them. You’d find a way to get someone inside the leading corporation researching a cure - the same company that seems suspiciously unwilling to share any knowledge it possesses about the sickness, even with its own workers. You might even reach beyond human means to help them. And when the inhuman comes in such gorgeous packaging, it might be hard to resist! Join Mona, K2, Rina, Mika, and Shinobu as they fight against the Return Syndrome, dark company secrets, and enemies (both human and inhuman). There may even be a dragonslayer in the mix!

Source: www.anisearch.com/manga/4977