It keeps coming back because his immune system is being stressed by his probably running himself ragged both physically and emotionally with worry for his wife. It's not some whoo-ha over-complicated mystery that only microbiologists/doctors/rocket scientists can understand.
I'm sure you'd recommend a whole big list of supplements etc. needed to get his body 'properly' back on track, based on your 'understanding' of how the body really works. *cha-ching*
But you see, without the parasites draining you dry of nutrients, it really is this simple.