The adult son of anti-vaxxers has claimed parents today have "no excuse" for not vaccinating their children after he became seriously ill when he contracted measles at the age of 30.

Joshua Nerius, from Chicago, Illinois, had no idea he wasn't vaccinated until he came down with the highly contagious disease in 2016.

The software product manager told CNN he went to see a doctor to get some antibiotics after developing a rash and a high fever.

But Nerius continued to get sicker, and it wasn't until he went to the emergency room he found out he had measles.

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When the doctor pointed out he had all the symptoms of measles, Nerius sent his mum a text asking whether he had been vaccinated.

She sent back a thumbs-down emoji.

He told CNN the preventable disease ravaged his body for months and took a "serious toll" on him.

He became so weak he couldn't walk on his own, and he lost more than 11kg.

Nerius came to the terrifying realisation of just how easily measles could be spread when he realised where he became infected.

He went to his sister's graduation at the Northern Illinois University College of Business and one of the attendees, from outside the US, was infected with the disease.

"I didn't interact with anyone at that graduation besides my own family, so it was literally just me walking by someone," he said.

Measles can be spread just by breathing in the virus that has been coughed or sneezed into the air by an infectious person. Measles can be contracted by just being in the same room as someone with the disease.

Symptoms include fever, tiredness, cough, runny nose, sore red eyes and a rash.

A third of people who get measles experience complications like ear infections, diarrhoea, and pneumonia.

People can be infectious even before their own symptoms start, which means they may not even be aware they are putting others in danger.

Nerius said the thought he could have unknowingly infected countless other people scared him.

The day after his sister's graduation he attended a technology convention in Las Vegas.

There were thousands of people at the event, but at that point, he had no idea he had been infected.

"I was shaking hands with hundreds of people a day. I wasn't contagious yet, but it's sobering to think if the timing had been just slightly different, how many people I could have infected?" he said.

Despite the terrifying ordeal, Nerius says he doesn't blame his parents for not vaccinating him.

They were believers in alternative medicine in the 1980s, and he said they didn't have the internet to be able to fact check the lies they were told.

But he said there were no excuses for anti-vaxxers today, as it had been proven time and time again how dangerous not vaccinating could be.

"The science on this has been settled. It's been solved. When I look at where we are today, with people who are wilfully deciding to ignore the facts, it really frustrates me," he said.

"I just don't understand the mindset of people who want to spread fear."