Susceptibility to colds requires a weakened immune system
Healthy people with normal immune systems are highly susceptible to cold virus infection once the virus enters the nose. In volunteers studies, approximately 95% of normal adults became infected when virus was dropped into the nose
Of people who become infected, only 75% develop symptoms with a cold. The other 25% have virus growing in the nose but have no symptoms. They have an "asymptomatic infection".
Why people sometimes become infected but do not develop cold symptoms is a mystery. One clue is that in such instances the person may not be producing the normal amount of certain inflammatory mediators, the natural body chemicals which cause cold symptoms. If this theory is correct, then people with active immune systems may be more prone to developing cold symptoms than people with less active immune systems!
Central heating dries the mucus membranes of the nose and makes a person more susceptible to catching a cold
As discussed above, a cold virus does not need the help of dry mucus membranes to initiate a cold once it enters the nose
The nasal mucus membrane is very resistant to the effects of low humidity. Volunteers placed in chambers where the humidity was dramatically lowered (9% relative humidity, such as found in a desert) still have normal clearance function of the nasal mucus membrane. Low humidity makes the nose feel dry but the mucus membrane still continues to work normally.
The cold season in the United States typically begins in late August and early September at a time when temperatures are still moderate and central heating is not being used. September is the time of a major common cold epidemic despite people not being exposed to the drying effects of central heating.
Becoming cold or chilled leads to catching a cold
As discussed above, almost everybody becomes infected whether they are chilled or not, if cold virus is dropped into the nose.
One study has looked at this question. It was found that colds were no more frequent or severe in volunteers who were chilled than those who were not.
Having cold symptoms is good for you because they help you get over a cold, therefore you should not treat a cold
Approximately 25% of people who get a cold virus infection do not develop symptoms and yet they get over the infection as well as people who do have symptoms.
The nose can only respond to irritative events such as a cold virus infection or dust or pollen entering the nose in a limited number of ways. Sneezing and nasal secretions are useful in removing dust and pollen from the nose but do not eliminate cold viruses since the virus is multiplying inside the nasal cells where it is safe.
Nose blowing propels nasal secretions into the sinus cavity. Nasal secretions contain viruses, bacteria, and inflammatory mediators all of which are able to produce inflammation in the sinus cavity. This may lead to secondary bacterial infection.
Nose blowing, sneezing, and coughing benefit the virus by helping spread it to other people.
Commercially available and FDA approved cold treatments are safe and effective. It makes sense to use them because they benefit the cold sufferer and may help prevent the spread of colds.
Drinking milk causes increased nasal mucus during a cold
Milk and mucus may look alike, but milk is digested like any other protein and is not specifically converted into nasal mucus.
An Australian study was actually done in volunteers to address this question. It showed that people drinking lots of milk had no more nasal mucus than those not drinking milk.
You should feed a cold
The origin of this old saying is obscure. There is no scientific evidence that excess eating will cure a cold.
On the other hand, eating tasty food will not make a cold worse and may help the cold victim feel better.