It turns out that what I was suffering last week in Istanbul was not the effect of a bang on the head (though that may have contributed perhaps) – I had shingles.
Shingles is caused by the same virus as chicken pox (Varicella zoster). If you had chicken pox as a child, you are probably home to the virus, which is kept in check by your immune system. But if you are old or stressed, or your immune system is compromised by another illness, the virus can strike. There is a theory that the decline in chicken pox infections in children (since immunisation became common) has meant that older people are less exposed to chicken pox and their immune systems lose the power to fight the virus, leading to an increase in cases of shingles.
It is painful, as I know now for a fact. The book says “very painful”, and the doctor who diagnosed it on Monday said I must be stoical to have endured that without demanding medical attention. Perhaps, if I hadn’t been distracted by thinking that the bang on the head caused it, I might have seen a doctor earlier. The earliest symptom was a headache and inability to concentrate (a very serious issue for a mathematician).
Anyway, if I seemed to be less responsive than usual for the last couple of weeks, that is my excuse. And here’s hoping that there are no long-term effects!