I caught Giardiasis in Ghana many years ago. Stupidly brushed my teeth at the cold tap in the hotel in Accra. (Bottled water please, never ever the tap!) My face literally went grey, most odd. Constant loo-visits, terrible abdo pain. Took two courses of antibiotics to get rid of it.
In most West African countries, one needs a current certificate of immunisation against Yellow Fever in order to be permitted entry. I had my injection at our local surgery, and the nurse told me I might have some 'mild flu-like symptoms'.
That night, I woke up flat on my back on the kitchen floor in my nightdress (no idea how or why I'd got there!) surrounded by several bemused and worried-looking cats. Overhead flew rather evil-looking monsters grinning maliciously at me. (hallucinations) I was sweating like a fountain. 'Flu-like'? Hmmm...
Fortunately, the regulations now state that one Yellow Fever jab is for life.
I wonder if there are any adverse reactions to the Covid vaccination?