Sunni Mariah was at her doctor’s office in Colorado when she noticed a sign up on the wall, promoting vaccination. So she took a photo and posted it on Facebook, commenting that the sign was “throwing some serious shade”.

Within a few days, the post had been shared more than 130,000 times, and had sent anti-vaxxers into a fury. What Mariah didn’t know was that the words on the sign had been written by a doctor from Australia, Rachel Heap.

Dr Heap is an intensive care specialist who works in the Northern Rivers region of NSW. In her spare time, she politely and respectfully does her best to correct misinformation put out by “professional anti-vaxxers”. It’s tough work, but she keeps at it.

“The ghosts of the people that I’ve looked after keep driving me,” she tells Mamamia.

Those “ghosts” are her patients who have died, or suffered horribly, from vaccine-preventable diseases.

In a six-month stint working in a kids’ intensive care unit, Dr Heap looked after babies with whooping cough. Two stick in her memory. There was one baby who spent six months in intensive care, and may have been left with permanent brain damage.

“The other one didn’t make it,” she says sadly.

“Those are two of my ghosts, but I’ve got countless, endless other ones, because people don’t count the adults. The 28-year-old guy who died of late complications of whooping cough. The people who’ve died of influenza – young, healthy people as well as the older and more frail. Name a vaccine-preventable disease, other than polio, and I’ve seen it directly.