It started with rumors of children fainting or vomiting after they received a vaccination against the polio virus in a village in Pakistan's northwestern province of Khyber Pakhtunkhwa.
Then clerics at local mosques in the region blared warnings through loudspeakers, ordering parents not to let health workers immunize their children against the deadly disease.
Meanwhile, anti-vaccination propaganda videos rapidly went viral on social media, with one claiming children had been "poisoned" by the drops.
Rumors originating from a suburb of the provincial capital, Peshawar, then claimed that children were dropping dead after receiving the vaccine.
As the rumors spread, thousands of panicked parents rushed their children by car, motorcycle, and foot to major hospitals in the city, forcing the stunned health facilities to declare emergencies.
Panic then turned into anger, with one mob burning down a local medical clinic in a Peshawar suburb.
The rumors turned out to be wildly exaggerated. Health officials said only several children out of the 25,000 rushed to hospitals were suffering from vomiting or stomach pain; there were no deaths.
The dramatic events of April 22 highlighted the major obstacles to eradicating polio in Pakistan, one of only three countries, along with Afghanistan and Nigeria, that suffer from the disease, a childhood virus that can cause paralysis or death.
Authorities arrested members of the mob that burned down the clinic and detained those behind the propaganda videos. The health minister went on television in a plead to parents to convince them that the vaccines were safe.
But the damage was done. The mass panic halted the April 23-25 immunization drive in Khyber Pakhtunkhwa, dealing a fresh blow to ongoing efforts to finally eliminate the disease from the deeply religious and conservative South Asian nation.
Many residents of the poor, largely rural Khyber Pakhtunkhwa have long been suspicious of the vaccine, with conservative Islamic clerics and militants claiming it is a Western conspiracy to harm or sterilize children.
Meanwhile, on April 27, Pakistani health officials announced they had suspended the anti-polio drive across the entire country following the killings of a health worker and two policemen escorting vaccination teams.
Riaz Khan was at work in Peshawar when he received a call from friends living in the Mashokhel suburb of the city.
"They said some children have died there after taking the vaccine," he said. "I got so scared for my children that I couldn't even think about the authenticity of the news."
The 35-year-old hurried home to his children, who had received the vaccination that day, and rushed them to the hospital.
"At the hospital, the doctors told me that there was nothing wrong with my kids," Khan said.
Muhammad Asim, an official at the Lady Reading Hospital, one of three major health facilities in the city, described what he said was "a nightmare."
"For the last 15 years, we have been through many emergencies because of bomb blasts and terrorism-related casualties, but this was a nightmare," he said.
Asim said the 500-bed hospital was overwhelmed with around 5,000 children and their families in the first 12 hours after the rumors spread.
"They literally choked our system," he said, adding that the three major hospitals in Peshawar were flooded with more than 25,000 children within 24 hours.
"All our doctors and nurses were trying to assure the worried parents that nothing had happened to their kids," he said. "We put announcements by well-known doctors on social media to calm the people, but it was like no one was ready to hear or believe it. I personally asked hundreds of kids and they told me that they are feeling just normal."
Dr. Shabeer Ahmad, a coordinator of the anti-polio campaign at the provincial health department, said they had decided to add Vitamin A to the polio vaccine to help malnourished children.
He said if taken on an empty stomach, Vitamin A can cause vomiting or stomach pain.
Ahmad said this happened to a few children in Mashokhel, where he said angry parents set fire to a local health clinic that was administering the vaccines. Nobody was hurt in the blaze.
"We added Vitamin A because it strengthens the immune system and helps the vaccine to be more effective," said Dr. Akram Shah, the director of the Expanded Program on Immunization (EPI), a provincial body. "But it backfired. We had to stop the campaign in many areas."
Shah said Pakistan has had a history of opposition to immunization efforts and "propagandists utilized the situation and took it to a new level."
Despite the obstacles, Pakistani health workers, together with the World Health Organization (WHO) and other international aid groups, have immunized millions of children across the country since 2012 with more than 100 rounds of the vaccination drive.
The effort has brought the number of polio cases to eight, compared to more than 300 in 2014.
Public health studies in Pakistan have shown that maternal illiteracy and low parental knowledge about vaccines -- together with poverty and rural residency -- are factors that most commonly influence whether children are vaccinated against the polio virus.
Another factor is that conservative Islamic clerics and militants in the region have spread propaganda claiming the vaccine sterilizes young boys.
Anti-vaccination propaganda has also been fueled by distrust of Western governments who fund vaccine programs -- particularly after the CIA reportedly staged a fake hepatitis-vaccination campaign in 2011 to confirm the location of Al-Qaeda leader Osama bin Laden, who lived in and was killed by U.S. SEALs in Abbottabad, Pakistan.
Since then, some clerics have even issued fatwas saying that children who become paralyzed or die from polio are "martyrs" because they refused to be tricked by a Western conspiracy.
Pakistani militants have also propagandized that Western-made vaccines contain pig fat or alcohol, which are both forbidden in Islam.
Militants have kidnapped, beaten, and assassinated dozens of vaccinators or their armed police escorts in recent years in a bid to stop local anti-polio campaigns.
A police officer was shot and killed by gunmen on April 23 while on his way to join a polio team in the northwestern district of Bannu. A day later, gunmen shot and killed another police officer guarding a polio-eradication team in the remote Buner district.
A female polio worker was shot dead by gunmen in the neighboring province of Balochistan on April 25.