Since having children, I can't fight off illnesses the way I used to. Colds and coughs turn into sinusitis or chest infections, while the rest of the family get away with the mild version of whatever is doing the rounds. How would I know if I were immune deficient?

It would be easy for me to write that you may be overtired looking after the children, and that this may have affected your ability to fight infections. But immunity doesn't work that way. For example, when the Spanish flu hit the forces in the first world war, it was the fit, healthy American GIs who died from it, not the war-weary, undernourished and depressed European soldiers. So there isn't a simple explanation. As you have presumably recovered from your sinusitis and chest infections, it is unlikely that you are 'immune deficient'. If you are worried, however, your doctor can check.

Stress raises your heart rate and increases contractility, just like exercise. Purely from a cardiovascular point of view, would half an hour's stress a day be as good for your heart as half an hour's running?

Stress does nothing for muscle tone, and I don't know how you would switch it off and on. In the long-term, repeated stress will increase blood pressure and lead to excessive acid production in the stomach - which is why peptic ulcers are directly related to stress levels. Regular exercise leads to a lower resting blood pressure and heart rate, while regular stress raises both. I'd stick to exercise and avoid stress.

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