He was a human calculator adept at arithmetic calculations and could count the number of bricks in a building at a glance. He was also the world’s first person to be diagnosed with autism.

On Thursday, American Donald Triplett made headlines again, when he died of cancer at the age of 89.

His nephew told US media that he died peacefully at home in the small town of Forest in the south-eastern state of Mississippi.

“He was just a joy to be around. He’d bring a smile to your face every time you get around,” said Mr Oliver Triplett, who was with his uncle at the time of his death.

The late Mr Triplett was first examined by Austrian child psychiatrist Leo Kanner in 1938 when he was five years old.

About five years later, he was referred to as “Case 1” in an article “Autistic Disturbances of Affective Contact” published in a journal by Dr Kanner.

The Johns Hopkins Hospital physician identified some basic symptoms in the children, including a preoccupation with objects, an insistence on consistencies and some language deficiencies.

Previously, children who exhibited similar signs may have been described as “feeble-minded or schizophrenic”, according to The Embryo Project Encyclopedia, a digital publication that records topics linked to developmental and reproductive biology.

Mr Triplett, known as Don to most people in his 5,000-strong Forest community, was known for his apparent gifts with numbers and memorisation, according to his entry in the Encyclopedia Britannica.

Numerous media reports cited how people in neighbouring towns had heard of him because as a teenager, he could calculate the number of bricks of a building in his high school simply by taking a glance. He could also effortlessly solve complex mental arithmetic sums and memorise Presbyterian church doctrine.

Mr Triplett was a graduate of an in-state liberal arts institution, Millsaps College, in Jackson, earning a bachelor’s degree in French. He worked at the Bank of Forest for 65 years.

“Don was a remarkable individual,” Bank of Forest chief executive officer Allen Breland told American broadcaster PBS.

“He was in his own world, but if you gave him two, three-digit numbers, he could multiply them faster than you could get the answer on a calculator.”

Mr Triplett, who was not married, also enjoyed playing golf and making solo trips around the world.  

News of his death was met with messages of condolences from local politicians and the local bank association, a sign of his standing in the Mississippi community.

Ms Zoe Gross, director of advocacy for the Autistic Self Advocacy Network, praised Mr Triplett for setting an example to all on the autism spectrum, and called for more to help people with developmental conditions.

“Obviously our society has made huge strides in the recognition and diagnosis of autism since Don Triplett was diagnosed as a child,” she told USA Today. “But there is much more that we still need to do to provide the necessary support to all autistic people, including accommodations in school and at work, and personal care services.”

Mr Triplett’s story was chronicled in a Pulitzer-nominated book In A Different Key, which led to a documentary with the same name, as well as other medical journal articles.

His nephew Oliver sought to offer hope to parents of children who are born different.

“They can see Don and a community who embraced him,” he told the New Orleans Advocate. “As a whole, Forest encouraged him and accepted him. It gives people who have children on different levels of the spectrum hope that their children can live happy and full lives.”