Relatives of the boy, called Rahmadullah, became concerned about him after he became seriously unwell at his family's farm in the south-eastern corner of Helmand province, near the Pakistan border.
His father, known as Nicknazer, took the boy to a small local American base, Forward Operating Base Dwyer, where doctors were at first stumped by his unfamiliar symptoms.
They were obliged to pull out text books before they diagnosed tetanus, now virtually extinct in the UK, and advised immediate evacuation to the larger British-run medical facility at Camp Bastion.
Lieutenant Colonel Andy Johnston, a Royal Army Medical Corps consultant physician who treated the youngster, said:
"When Rahmadullah arrived here at Bastion he was having severe and painful muscular spasms which were affecting his whole body and interfering with his breathing.
"He also developed pneumonia and had to be put on a ventilator for nearly two weeks. He was extremely unwell."
The medic, who was previously based at the Royal Centre for Defence Medicine at Selly Oak in Birmingham, added:
"This was very much a team effort to get this little boy through his life-threatening illness.
"If it wasn't for the efforts of the doctors at Dwyer and the intensive care unit [ICU] team at Bastion, Rahmadullah would have died.
"Fortunately, he has gradually improved to the point where he is now well enough to go home.
"Just to see the expression on his face when his father came to pick him up made it worth it."
Rahmadullah spent more than two weeks at the hospital where medical staff were said to have become 'very attached' to him.
They believe his tetanus developed from a tiny cut on his foot.
He has now been discharged from hospital and, although he was 'weak', he was expected to be able to walk unaided in a matter of days.
Lt Col Johnston said:
"We see a lot of children in ICU, usually as a result of road traffic accidents or military trauma. Often they die from the severity of their injuries so it is very gratifying to see this little boy get better."
Speaking through an interpreter, Rahmadullah's father said the little boy would have died without treatment which he would ordinarily have been unable to afford:
"I saw my son getting weak and coughing. I took him to local doctors but they couldn't do anything, so I took him to the local military base.
"There I was told he was very ill and needed to come to Bastion," the farmer, who is thought to be in his thirties, explained.
"You can see how happy he is. The doctors and nurses have been fantastic. I had no money to spend on local doctors who are very expensive.
"My son would have died if I had not brought him here, so I am very pleased."
There is currently no vaccination programme in Afghanistan against tetanus and medics estimate the disease to be fairly rife, particularly in the more remote village communities.