Sheepies said:
Especially with a weak or compromised immune system.
in younger, healthy people, it's the opposite. cold and flu viruses are mostly harmless, but they exploit the immune system's symptomatic response for their spread and younger folk, with their less mature and often overly aggressive immune systems, do that job better than older folk and they therefore tend actually suffer more by their having more severe symptoms. it's the symptoms which ail thee, not the virus.
malnutrition and adrenaline will likely prolong a cold or flu, by deferring the symptomatic process which, once infected, you have to go through. usually though, keeping well-fed, warm and rested, that'll be 3 to 5 days. 'rested' also includes learning not to cough and sneeze your head off as this can cause more inflammation and cuts to your already inflamed throat and nasal passages, leading to secondary bacterial infections.
heat, inhaled as moist heat from a pan of hot water or dry heat from a heatlamp or toaster, will ease some of the symptoms as well as killing any viruses and bacteria along the nasal passages and throat. it's trial and error as to which works best for you, and it may a mix of both (i often find benefit in going from a moistened state directly to a drying out state). further benefits of heat are a bit speculative, but well-reasoned [as to the virus killing effect] and often 'grandmother cures' for colds and such (hot baths, feet in a bowl of hot water, etc).
not had a flu-jab for years. i have a naturally aggressive immune system and the last time i had a jab (for key-personnel, at that time), it triggered a response so that i may as well have had the flu.