Time of year?

Well Mike has got to go for a chest x-ray tomorrow to see if the antibiotics did there job and to check there is something else causing the cough. Ironically it would seem that just like two years ago, he may now be going down with a sinus infection. He sounds a bit funny when he talks, (I know he always sounds a bit strange in the head but that’s why I love him) and it reminds me of when he was ill and in hospital for weeks and weeks back in 2010. On that occasion he was admitted on May 7 after a couple of weeks of being off colour and then a week of gradually going downhill until he was so ill he couldn’t swallow or even speak clearly.
It has got me thinking that may be it is caused by a reaction to a certain type of pollen. Is it a particular tree that flowers the end of April? Or is it simply coincidence or bad luck?

Polite answers will be gratefully received.