As Yennifer says we build up immunity by coming into contact with viruses and bacteria and our immune system kicks in to fight them and retain some immunity to these specific bugs.
How well we do this depends on our immune system which is influenced by our general health, whether we have certain medical conditions, such as diabetes, and importantly, age. Immune system is not as efficient when we age, that is why sero conversion for flu vacc doesn't always work in the elderly.
For a virus such as Covid 19 which is new, I don't really understand how being resilient to plenty of other bugs (ie ? present in a dirty house) would help, apart from showing your immune system is functioning.
Perhaps someone can enlighten me?
WORD PAIRS -APRIL 2026 (Old thread full )
Air Conditioning units ,any advice please?

