(Andrew Burton/Getty Images)

The reason Bella Santorum was admitted to Children's Hospital of Philadelphia Saturday night is because she has pneumonia in both of her lungs, a campaign aide told ABC News.

The 3-year-old, who has the serious genetic disorder Trisomy 18, is not on a ventilator, though, which is a very good sign. The family has medical equipment in their Virginia home and because of the risk of infection in hospitals they try to keep her out unless it is absolutely necessary.

She was in and out of hospitals the first year of  her life, but since then - aside from besides being admitted for surgeries - she has not spent any time in the hospital before Saturday, Rick  Santorum has said on the campaign trail.