The Chicago Fire Department sent 10 ambulances to the airport, not knowing how many people they would need to treat. Paramedics evaluated about a dozen passengers and took four of them in fair condition to a hospital.
There were about 138 passengers on board the flight. The people who got sick were all traveling with the same tour group.
Once the plane landed at Chicago O'Hare International Airport, passengers switched planes and continued on to LAX. The flight finally arrived at LAX at about 1 a.m. Thursday.
Some of the people with the tour group apparently got sick before they boarded the flight in Boston.
"As I was getting on the plane there were a couple of passengers in front of me who said that they had been traveling with a group, and that there were 35 of them. And out of the 35 there were already seven who had gotten sick, and that they had gone to the hospital and they had been, you know, throwing up before. And I was kind of concerned that they were getting on the plane in the first place," said passenger Kaitlen Reyes.
It's still unclear what made the passengers sick. A Chicago Fire Department spokesperson said at least some fell ill after smelling vomit on the plane.
United Airlines says it was not serving food on board the flight.