What made Jo turn as white as a sheet?

Jo became exceedingly pale when the doctor came out and told the sisters it was time to summon their mother..... Beth was gravely ill.

Hannah nodded without speaking, for her lips twitched nervously, Meg dropped down into a chair as the strength seemed to go out of her limbs at the sound of those words, and Jo, standing with a pale face for a minute, ran to the parlor, snatched up the telegram, and throwing on her things, rushed out into the storm.