What causes a child to become sick literally all of a sudden?

Depends: It is often a virus, but occasionally it is a severe and dangerous issue, so i would have them immediately checked out if you are not sure.

PATTERN OF ILLNESS: Many childhood illnesses go through an incubation phase.The germs they were exposed to are replicating in their system, but few if any signs are evident. When the germ load reaches a certain point, the germ itself (virus) or chemicals generated by germs enter the blood and fever rises and other signs are seen. Many are not as aware of their body or verbal enough to tell you early when ill.