Stress, by liberating adrenaline and activating the spleen, can counter infection.

“that in a fight, it's rare that you ever feel pain when you get hit, I've experienced that, but later it hurts a lot. It turns out that when you stimulate these pathways that activate, in particular, the adrenals, the adrenal gland liberates norepinephrine and epinephrine and the brain does as well, it binds to what are called the beta noradrenergic receptors. This is really getting down into the weeds, but the beta noradrenergic receptors activate the spleen which liberates cells that combat infection and it's anti-inflammatory.”