"For all practical purposes the difference between a cold and 'flu is severity."

Absolutely not the case. Unless you're talking about a cold and man flu. The two illnesses are quite distinct in speed of onset, symptoms and severity. I've had flu five times in my life. No runny or blocked-up nose, no sneezing. Aching limbs, tender skin all over, fever, nausea, severe headache, unable to get out of bed, utter exhaustion. If you're lucky. A dose of flu can down you in an hour from your feeling just fine.