Children were among the 42 migrants hospitalized after officers in Croatia pulled over a van packed with 67 people inside, authorities said Sunday.
Doctors said the van's occupants were suffering from carbon-monoxide poisoning and some were unconscious.
The country's health minister, Milan Kujundzic, said the migrants were treated at local hospitals and are now safe.
"Most will be released but three or four will remain hospitalized for observation," Kujundzic told the Hina news agency.
Kujundzic added that two children, ages 10 and 12, were among the migrants crammed in the van. Many of the migrants told officials they were cold, hungry and some said they hadn't eaten in five days.
Sky News reports that many of the migrants were from Afghanistan and Pakistan.
Police stopped the van with British plates Saturday evening as it was traveling near the town of Novska, which is by the Bosnian border. Officials said the van was headed west toward the capital and likely into western Europe.
Authorities noted that some of the migrants were even packed into the van's cargo compartment.
Two Bulgarian citizens are under investigation for smuggling the migrants into the country.
The Associated Press contributed to this report.