The dynamic Wasps number eight ended up missing four months of last season after the infection led to discitis and then a catalogue of misfortune. Vunipola contracted pneumonia while he was in hospital and he also suffered severe burns in a hot-water bottle accident as he tried to ease the pain in his lower back.
"I was in hospital for quite a long time so to be sat here now (alongside Mako and in the England squad) is quite unbelievable," said Vunipola, who will join his brother at Saracens next season. "I am not sure I am about to be an England international just yet, but the prospect of it is really exciting."
But when he finally returned to action, Vunipola quickly developed into a key figure in Wasps' successful battle for Aviva Premiership survival.
The 20-year-old's form at the start of this campaign saw him fast-tracked into the England training squad for the RBS 6 Nations and now he is eyeing a Test debut.
He added: "I had discitis. I got pneumonia and then I burned my back with a hot water bottle so I was in there for quite a bit longer. I fell asleep on the hot water bottle and forgot to put the outer cover on.
"When I came out of hospital I had to go on antibiotics and go in for regular checks."
The England coaches are keen to let the 19stone 12lb younger Vunipola loose on the international stage but he is having to bide his time and remain patient.
An untimely ankle injury, illness and toothache sidelined him at the start of the Six Nations and cost him the chance of a place on the bench for England's win against France.
Tom Croft's return to the squad this week, after nine months out with a neck injury, is likely to force Vunipola to wait a little longer before he makes his England debut, but Mako is expected to be given his first Test start against Italy on Sunday in place of Joe Marler after consistently making an impact off the bench.
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