Strep throat is a common childhood illness, but in a few children this bacteria infection can lead to alarming, lasting changes in behaviour. Scientists are now starting to unravel how it affects the brain.

On Charlie Drury's eighth birthday, his life changed forever.

It was November 2012 and the day had all the usual birthday fun – cake, presents and themed party food. But as the day wore on, Charlie's mother, Kate Drury, noticed something strange about her son's behaviour. It started with an eye twitch and then he kept sniffing his hands. Later in the day, he developed a fever. Drury took him to an urgent care clinic near the family's home in Illinois, US, where he was diagnosed with Streptococcal pharyngitis, also known as strep throat.

In the weeks that followed, Charlie's health deteriorated in ways that baffled Drury. He became so sensitive to smells that Drury had to stop cooking in the house. He developed extreme separation anxiety, yet his own mother's touch caused him to scream and scrub vigorously at the spot where their skin had met. He barely slept, became anorexic and refused to bathe. He hurled objects around their home. He hallucinated. He struggled to read and write. Drury barely recognised her son, who had previously been a promising student and athlete with no behavioural issues.

"I lost my child in a day," Drury says.

After more than a month of agony and confusion, physicians diagnosed Charlie with a little-known condition called "paediatric autoimmune neuropsychiatric disorders associated with streptococcal infections", or Pandas.

Children with Pandas experience a stark and sudden onset of tics and/or signs of obsessive-compulsive disorder (OCD), with symptoms often appearing seemingly overnight. Just as suddenly, a child with Pandas may develop a litany of other issues including regressions in developmental or motor skills, aggressive and erratic behaviour, mood swings, pain, irritability and sleep disturbances, among others.

The condition's exact prevalence is hard to pin down, but experts agree Pandas is not common. One estimate says one in every 11,800 children might develop Pandas or a related condition called paediatric acute-onset neuropsychiatric syndrome (Pans) in a year, although the numbers do vary dramatically from study to study. They do, however, seem to be more prevalent among boys than girls, according to data from an international patient registry, and strikes children before puberty.

"For obvious reasons, parents are completely terrified" when these issues emerge, says Shannon Delaney, a child and adolescent neuropsychiatrist in private practice who treats patients with the condition in New York City, US. "I frequently hear them describe it as, 'It feels like my kid is not there. It feels like they're possessed'."

Researchers from the US National Institutes for Health (NIH) first described Pandas in the 1990s. Even at that time, the idea that streptococcal infections could cause neurological complications was not new. Doctors in 1894 had described patients with a neurological condition called Sydenham chorea after developing throat infections. Around 100 years later, a group led by Susan Swedo, then chief of the paediatrics and developmental neuroscience branch at NIH, published evidence that suggested strep infections were also linked to the startling and abrupt appearance of neuropsychiatric symptoms that are typical in Pandas patients.

How could strep throat – a routine illness contracted by hundreds of millions of children around the world each year – cause such an intense response among a small subset of kids?

This has been the subject of active scientific debate in the years that have followed. But Swedo, who is now chief science officer at the Pandas Physicians Network in the US, sums it up like........

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