Measles are surging in Texas, with experts warning no sign of slowing until at least May. The outbreak has now spread to its third state, Oklahoma.

On Tuesday, health officials in Oklahoma reported two “probable” cases in the state that appear to be linked to the ongoing outbreak in Texas and New Mexico. According to the Oklahoma State Department of Health, the two people developed measles symptoms after exposure to cases associated with the Texas and New Mexico outbreaks.

Why do health experts fear the West Texas measles outbreak may be much larger than reported? For some, it’s a question of simple math.