Stephan Schneider, the German doctor working in Catalonia who diagnosed the six-year-old, had previously warned of the possibility of the disease cropping up again in a blog post in 2011 after reports of a case in France.
El médico alemán que lo diagnosticó: «El caso de Olot confirma que la difteria es una ruleta rusa con balas reales» http://t.co/5B9XTbonhl— ABC.es (@abc_es) June 11, 2015
In an interview with ABC published Thursday, the doctor gave his thoughts on the boy's parents deciding not to vaccinate their child.
"They were simply poorly informed," he said. "Now, his parents and all the parents in the world are already aware that the disease is a real risk. This case has demonstrated that diphtheria is Russian roulette with real bullets."
The boy identified as Pau from Olot in Catalonia is the first case in Spain of the disease recorded in Spain since it was eradicated there nearly 30 years ago.
Pau reportedly remains in serious condition with heart and lung problems as well as kidney failure and is being kept alive connected to three different machines.
Diphtheria is an illness caused by the bacterium corynebacterium diphtheriae that usually affects young children and the elderly.
It is spread through the respiratory tract via coughing and sneezing and has a mortality rate of one in ten.
The case has stirred heated debate about whether parents should be legally obliged to vaccinate their children.
The boy's parents have expressed deep guilt and regret for not vaccinating Pau, saying they felt deceived by the anti-vaccination movement.
A 'real danger'
Schneider explained the reaction of Pau's parents when he informed them of their child's diagnosis.
"They were worried, which is logical. The first thing they told us was that their child was not vaccinated," he said.
Eight of the boy's school friends have tested positive for the illness, though because they were vaccinated they have not become sick.
An adult who had been in contact with the boy has also tested positive for diphtheria, officials reported on Thursday.
And yet 47 children in schools where Pau lives still remain unvaccinated, the Catalan health minister announced on Friday.
Since diphtheria had not been seen in Spain for so long, officials had to fly in medicines from Russia.
Schneider said that because he had not seen a case of the disease before, he had to search for images online when the boy was first brought to him. He said the fact that his grandfather had suffered from diphtheria, the possibility had always been at the back of his mind.
"Diphtheria has always been in my subconscious, I always thought it was a real disease, a real danger," Schneider said.
Some have argued that vaccinating children should be legally required for parents, but Schneider believes that dialogue is more important than legislation.
"I appeal to the responsibility of individuals," he said. "I'm more open to debate. Legislation should always be the last option in a democratic culture."
Still, he had more critical words for the anti-vaccination movement that fears the effects of vaccines.
"Anti-vaxxers lie when they say to families that diphtheria no longer exists," he said. "I just ask them to take responsibility."