Cursed children are contaminated with the Gastrea virus. If erosion rate reachs 50%, they turn into Gastrea. Does cursed children erosion rate grow as time passes? Or they have to get more contaminated with other infections for the rate to grow?
The corrosion rate grows as they use their Gastrea ability. This is shown in the scene where Rentarou received Enju's test result.
The more they exerted their Gastrea abilities, the faster they will turn into Gastrea.