Flu.

I agonised long and hard when I reached the age of 60 as to whether or not to have 'the flu jab' each year. The negatives did not quite cancel out the positives and so I opted in.
Even the experts are unsure as to how effective these jabs are but as we apparently have 'Aussie flu' rampaging through the southern hemisphere. It is coming to the UK.
It is probable that the 'Spanish flu' which devastated the world in 1918/1919 would have killed only a fraction of the fifty million it wiped out had those people had access to a modern anti-flu programme.
Yes. I have read the conspiracy theories.
On balance - the limited protection offered by the jab seems a no-brainer. If not available on the NHS to you - it is quite inexpensive at ASDA.