I was coming home from the shops with my (at the time) 4 children, just about to unlock the door, when my son aged 3 suddenly went floppy, collapsed and limbs felt rubbery, like a bendy toy.
I went a few doors away where a nursing sister lived with her paramedic son whose car I spied on the driveway.
He stripped him, lay him on towels and his mum got ice, bowls of cold water and soaked them before swaddling him. Son rang ambulance station and he went in vehicle with him while I followed in car. Halfway through journey the ambo diverted to another hospital to stabilise him. In an ante-theatre he was laid out and some tests were done to see if co-ordination was returning. They decided to take him to Alder Hey by air ambulance, more than 150 miles away.
The paramedic and a doctor from the hospital went in the helicopter, I drove with children and, incredibly, a police escort (those were the days).
Son was kept in for a week, lumbar puncture was clear, diagnosis was otitis media.
The only sign of him being unwell had been earlier in the bank when he said he was hot and wanted to take his jacket off. 
Luckily, my then DH was abroad, otherwise he would have had a complete meltdown.