Well, well talking sense (although YOU saying something for a thousand times doesn't make it so! But in this case it is true).
However, you mention flu. Many think they've had flu but haven't really had the muscle aches, even your hair hurts. You REALLY know if you've had flu.
But the flu vaccine DOES help protect against infection (my doctor quotes 80% protection).
Flu jabs are available in Thailand for about 1000 Baht.