We believe there is a biological sex difference. I've been hearing this for about 8 years, Google it. They say the x chromosome is coded for more immune system boosters; women have 2 & men, obviously, have just the one. They also think testosterone, while excellent for daily tasks & power, might lend some immuno-suppressant qualities.
What is man flu? Genuinely curious, I always thought it was just a kinda tongue-in-cheek razz. Have you experienced differences while/after transitioning?
Being sick is just worse in a testosterone based body. Your symptoms are worse and you just feel more tired, mild colds aren't so mild anymore. And Wikipedia has an interesting page about it..
I'm thoroughly convinced that the reason women tolerate the flu better than men is that they're simply more used to feeling like shit every once in a while and just putting up with it. If you get periods every month, then feeling a little sick from a cold/sore throat is not a big deal in comparison.