Air carrier flight crew reports numerous passengers becoming sick on cross country flight causing diversion.

Narrative

Approximately 45 minutes to 1 hour after departure; when the crew meals were brought up; we were advised that we had a sick passenger onboard. The passenger apparently went into one of the aft lavatories and got sick in the sink. The Purser told us the lavatory had been locked off and the passenger was vomiting at their seat but they were ok to continue. About 1 and half hours into the flight we received a call from the Purser that we now had 4 passengers who were vomiting. We were in light to occasional moderate chop. The Purser seemed to think that there was no cause for concern and felt we could continue on to ZZZ1. The Flight Attendants seemed to think it was the turbulence or possible food poisoning that was making the passengers sick. My First Officer and I; however; were concerned that the number of sick people had quadrupled in such as short time and initiated a call to Dispatch and Medical. While we were getting patched through to Medical; we received a call from the Purser that the number of sick passengers had doubled and we now had 8 people vomiting at their seats and in the two working lavatories. We explained the situation and passenger symptoms to Medical and were told as long as they were just vomiting it was OK to continue on to ZZZ1 and to give the passengers ginger ale. Medical also informed us that if there were any significant changes; such as losing consciousness or if they started having diarrhea we should contact medical to consult. We asked the Flight Attendants to get more information from the sick passengers. We were able to determine that all the sick passengers were senior citizens and part of a tour group of 35. Upon the recommendations of Medical; Dispatch; and the information we were getting from the Flight Attendants we decided to continue on to ZZZ1. Approximately 30 minutes later; my First Officer and I noticed a foul smell coming from the cabin into the cockpit. Shortly after that and just past ZZZ we were informed by the Flight Attendants that we now had 12-14 sick passengers; all part of the group; and they were vomiting and having diarrhea. Based on that information; I made the decision to divert into ZZZ since the situation onboard was deteriorating rapidly and the passengers were elderly. We coordinated with dispatch and ATC to divert to ZZZ. We relayed information to Dispatch to have Medical responders standing by at the gate. We also attempted to have the Flight Attendant get more information about the passengers; such as the seat numbers of the sick passengers; but never received the information. After a normal landing; we taxied to gate and the aircraft was met by medical responders and air carrier personnel. Even though the personnel who met the plane did a great job; there was not one Supervisor in charge to handle the situation. We had all sick passengers deplane first followed by all members of the group. The group leader was trying to convince me that the sick passengers were ok to continue. Flight Attendants reported to me that she was in the aisle telling the group members to sit down; put their bags away; and pretend that they were ok or they would be taken off the plane. After much resistance the entire group deplaned and were held in the gate area to be checked by Medical responders. After the tour group deplaned; all other passengers deplaned. During their deplaning; a few passengers informed me that these passengers didn't feel well in. Some had overheard conversations in the gate area that 1 or 2 of the sick passengers had already gone to the hospital in ZZZ2. I was also informed that the Flight Attendant talked one sick passenger; I believe the first passenger to become sick; during boarding and had concerns that they maybe should not be on the flight; however; I was never notified. I only became aware of the situation 45 minutes to 1 hour after departure. We had to switch aircraft in ZZZ because of the vomit and human feces that was throughout the cabin. When we arrived in ZZZ1 almost 4 hours late; there were no Supervisors there to meet with the passengers. After a situation like that; it would have been nice to have someone meet the plane to make sure the rest of the passengers were alright and answer any questions or ease concerns they might have had. Overall I feel the group leader was more concerned about getting home than the health and safety of these poor passengers. Due to the irresponsibility of her actions she not only risked the safety of the flight but she showed utter disregard for the medical situation happening with the members of the tour group. My First Officer did a great job and because of that we were able to return to ZZZ quickly and get the sick passengers medical attention. Sadly; I think this situation could have been avoided.