The Common Cold
As we get older our immune system learns how to deal with more and more viruses as we generate antibodies each time we get a cold.
Can a cold kill you?
Yes! Babies and the very elderly can develop chest infections such as bronchiolitis caused by the RSV virus that can be fatal. You are also at risk take any medication that depresses your immune system.
Viruses travel in style
Similar viruses are found in both the southern and northern hemispheres and in temperate and tropical climates and this is because cold viruses are as mobile as their host and readily jump from one continent to another by jumbo jet.
A golden age for common cold viruses
The densely populated cities of the modern world provide ideal breeding grounds for common cold viruses. Modern transport systems whisk viruses around from city to city.
Infection usually occurs at home
Young children represent the main reservoir of common cold viruses and infection usually occurs at home or in the nursery or school. Adults with regular contact with children are most exposed to infection.
Common cold viruses are not very contagious
Despite the fact that very few of us escape from at least a couple of common cold infections each year, common cold viruses are not very contagious. Under laboratory conditions when healthy volunteers are kept with others who are suffering from common cold infections it has proven remarkably difficult to spread infection from one person to another 2.
Kissing is OK
Close personal contact is necessary for the virus to spread and the home and school are the places where spread most often occurs. The common cold viruses are not spread by contact such as kissing but appear to be spread by large particles expelled at close range by coughs and sneezes, and by contaminated fingers that pass the virus to the nose and eye. Your fingers can easily become contaminated with viruses by touching door handles etc. in public places. You may then touch your nose or eye and infect yourself. Tears from the eye drain via a duct into the nasal cavity and when we touch our eyes with contaminated fingers we pass viruses into the nose3!
How to avoid catching a cold
Become a hermit. If you are in contact with other people you are likely to get a cold as the viruses are so common.
Hand washing may help
Since cold viruses can be passed from person to person by hand contact or by touching contaminated surfaces such as door handles you can help prevent infection by washing your hands. Home studies have shown that hand washing can reduce the spread of common colds within the family4
When am I most infective?
The incubation period for a common cold is usually around two days before symptoms start. You are most infective when you have the early symptoms of sneezing, runny nose and cough. The viruses replicate in the cells lining the nose and they are coughed or sneezed out in droplets of mucus. We can also spread the virus on our fingers when we contaminate them with secretions from the nose. In order for you to spread the infection you need to have close and prolonged contact with other people and to cough or sneeze on them or pass on secretions from your nose via your hands.
The colds season
Colds occur all year round but there is an increased incidence during autumn and winter months.
Can a chill cause a cold?
There is no scientific evidence that chilling the body causes an increased susceptibility to infection or an increase in the severity of symptoms. However there is so much belief in the idea that chilling can cause a cold that it is possible that there may be a link between chilling and colds that has not yet been mimicked by scientists under laboratory conditions. One theory is that chilling of the body surface causes a pronounced constriction of blood vessels in the nose and that this may lower our resistance to infection. The idea is that when colds are circulating in the community many persons may be infected with a virus but not show symptoms. The chilling of the body surface then lowers resistance to infection in the nose and aids the viral infection. In this case the person has not caught a cold by chilling but activated a latent or sub-clinical infection that was already present in the nose5.
Why more colds when its cold?
Colds and flu are definitely seasonal, with more colds in the colder weather but there is no real agreement as to why colds are seasonal. Most textbooks state that there are more colds in cold weather because we tend to crowd indoors in poorly ventilated rooms. This crowding theory has been around for over a hundred years but it does not really make sense, as our cities are just as crowded in summer as winter. A new theory that has been put forward to explain the seasonality of colds and flu, and this theory puts forward the idea that our noses are colder in winter than summer and that cooling of the nose lowers resistance to infection. If the weather is freezing outside we wrap up in winter clothes but we still leave our nose exposed to the freezing air. Every time we breathe in we cool the nasal lining and weaken our local defences against infection. If this theory is correct then covering our nose with a scarf in cold weather could help prevent colds6.
Common Cold Viruses
Which viruses cause colds?
The symptoms of the common cold syndrome are caused by over two hundred different viruses. Rhinoviruses (nose viruses) account for approximately 30-50% of adult colds. Other viruses, such as coronavirus, respiratory syncytial virus (RSV), adenovirus, parainfluenza virus and influenza virus also cause the common cold syndrome. There is no way of identifying the nature of the virus infection from the symptoms as these are very similar across the whole range of viruses.
Incidence of colds according to virus7
Rhinovirus 30-50% Coronavirus 10-15% Respiratory Syncytial Virus 10% Adenovirus <5% Parainfluenza 5% Influenza 5-15% Enteroviruses <5% Metapneumovirus unknown Unknown 20-30% The metapneumovirus is s new common cold virus discovered in 20018. There may be more unknown colds viruses still awaiting discovery!
What is a virus anyway??
Viruses differ from bacteria in that they do not have a cellular structure and they are much smaller. Some scientists say viruses are not ‘alive’ because they need to replicate inside a cell such as those lining our nose. The rhinovirus is icosahedral in shape i.e. is a twenty sided structure with a protein shell surrounding a strand of RNA genetic code.
Viruses are really small
The rhinovirus is around 20 nanometres in diameter. To put that in perspective you could line up fifty thousand rhinoviruses end to end across one millimetre. Another way of thinking about the size of the virus is that a typical human cell in the nose would be around 20 micrometres in diameter and therefore around a thousand times bigger than the virus. It is difficult to imagine such a small virus causing the misery and symptoms of the common cold.
As a single layer you can lay 1.35 x 1012 rhinoviruses on one surface of a 1 Euro coin. That is one thousand three hundred and fifty billion viruses or over 200 for every man woman and child on earth.
How the virus gets up your nose
When the virus enters the nose it is probably accompanied by thousands of similar viruses all stuck together in a tiny blob of mucus which has been picked up from an infected person. Viruses do not float around as single viruses, they are always found in respiratory mucus from the nose. When you catch a cold it is interesting to speculate. Whose mucus has got up your nose?? A critical stage in infection is the attachment of the virus to nasal cells lining the nose.
The virus uses a docking mechanism
The mechanism used by the rhinovirus to attach to the nasal cells has been studied in some detail. The rhinoviruses have many depressions on their protein shell called ‘canyons’ and these fit on to a surface protein receptor on the nasal cell just like a docking port9 . The cell protein receptor is called an ICAM receptor (Inter Cellular Adhesion Molecule). Once attached to the cell the virus enters the cell and uncoats to release its strand of genetic code.
Colds and computer code
The genetic code of the colds virus is in the form of RNA (Ribo Nucleic Acid) and this is rather like a bit of computer code that is inserted into the cell machinery and which instructs the cell to construct new viruses. The strand of RNA contains around seven thousand nucleotides and is in the form of a long string which is tightly coiled inside the viral shell9. The normal operation of the cell is switched off by the viral RNA, and the cell produces thousands of copies of the virus. The viruses are released into the nose when the cell disrupts. These viruses soon infect adjacent nasal epithelial cells or are expelled from the nose in drops of mucus to start a new cycle of infection in another human nose.
Infection does not always mean a cold
Most viral infections in man produce no disease at all, they are ‘subclinical’ (i.e. no symptoms), despite extensive viral replication . The concept that the majority of viral infections pass unnoticed without any signs of disease is known as the ‘iceberg concept of infection’ as the classical and severe disease cases only represent the tip of the iceberg of infection10.
Colds are like an iceberg
The ‘iceberg’ concept of viral infection describes the general consensus by virologists that most infections in the community do not cause symptoms and pass without notice. At the very bottom of the iceberg we have the vast majority of encounters with common cold viruses in which the virus does not infect the nose or causes only minor symptoms such as short lived throat irritation and a couple of sneezes which are not recognised as a cold.
A stinking cold
The infections that we recognise as colds with severe symptoms are the minority and represent the tip of the iceberg of infection.
Factors which influence the immune response, such as stress may be important in determining whether we develop cold symptoms or not.
Get stressed and get a cold
There is much evidence that indicates that the stress of every day life can influence the susceptibility to infection. Experimental studies in which common cold viruses have been administered into the nose of healthy volunteers have shown that there is a link between recent history of psychological stress and susceptibility to infection. Stress appears to be associated with the suppression of general resistance to infection that leaves the person susceptible to infection. It is still not clear how psychological stress affects the immune system but the most likely link appears to be the increase in the release of corticosteroid hormones associated with stress, as corticosteroid hormones are known to decrease resistance to infection11.
The increased stress of modern city life may be one of the factors predisposing to the very high incidence of common cold infections in our crowded cities.
Common Cold Symptoms
Symptoms flare out from a pinpoint of infection
Common cold infections do not cause any visible damage to the lining of the nose. Sites of infection may occur as tiny pin points scattered over the lining of the nose and throat. The pinpoint of infection is represented as the tip of the flare and this triggers a cascade of events in the lining of the nose starting with the immune response and sometimes expanding to generate chemical mediators and symptoms.
Why do I feel tired and lose my appetite when I have a cold or flu?
The systemic symptoms such as fever, headache, muscle aches, tiredness and loss of appetite (anorexia) are caused by the release of chemicals from the white cells that fight off infection. The chemicals are called cytokines and they circulate in the blood to the brain to cause fever, tiredness and loss of appetite. This is the body’s way of telling us to rest and save all our energy to fight infection12. In general common cold viruses do not trigger a large response from the white cells but severe cold infections and flu can cause a lot of cytokine release and give us the typical ‘flu-like’ symptoms of fever, and tiredness.
Feed a cold, starve a fever?
The old saying ‘Feed a cold starve a fever’ is part of our folklore13. The origin of this saying may be related to the fact that any feverish illness is also likely to be associated with loss of appetite due to the actions of circulating cytokines released from white cells as discussed above. Most colds in the adult are not associated with fever and therefore there is not much loss of appetite.
Killer cells and antibodies
Viral infection triggers the immune response by the activation of white cells that act as killer cells or produce specific antibodies. It is the killer cells that overcome the infection rather than the production of antibodies.
Antibodies do not cure your cold
Antibody production starts two weeks or so after the infection and reaches a maximum after three to four weeks9. The antibodies help to prevent repeat infections by the same virus but do not protect against other viruses. Since there are over two hundred different types of cold virus, over the period of a lifetime one could have a couple of infections each year with each infection involving a new virus.
I have just had one cold can I get another right away?
Yes. The antibodies you generate against colds viruses are very specific and since there are around two hundred known types of colds virus you could easily encounter two different types one after the other.
What triggers the symptoms?
It is the immune system that triggers the symptoms and not damage caused by the virus14. The natural disinfectants produced by the white cells can cause damage to the cells lining the nose and this triggers the symptoms. Oxidising agents produced by white cells in response to viral infection may activate enzymatic pathways that lead to the generation of chemical mediators in a cascade of reactions that eventually result in symptoms.
Chemical mediators cause symptoms!
Common cold symptoms are caused by the effects of a complex soup of chemical mediators on blood vessels, glands and nervous tissue12. Mediators such as bradykinin, prostaglandins, tachykinins, histamine and cytokines are formed as components of the inflammatory and immune responses to infection.
Bradykinin is important
Out of the complex soup of chemical mediators responsible for the symptoms of common cold the peptide bradykinin has been proposed to play a dominant role.
Bradykinin can be detected in nasal washings taken from patients with common cold infections and intranasal administration of bradykinin in healthy volunteers causes, nasal congestion, nasal irritation, throat irritation and stimulation of nasal secretion15.
A bradykinin antagonist to block cold symptoms?
There is much interest in developing a medication that would act as a bradykinin antagonist as this would be a rational approach to treating common cold symptoms in the same way as antihistamines can be used to treat the symptoms of hay fever. If such a development is to take place it is likely to come from research into medications for the treatment of rheumatoid arthritis since bradykinin is also implicated in causing the pain and swelling associated with joint inflammation.
Symptoms start two days after infection
The time to onset of common cold symptoms can only be studied in persons who are infected experimentally with a common cold virus and in these volunteers symptoms typically start one to two days after infection.
Symptoms may last two weeks
Symptoms in patients presenting with common cold usually last for around seven days although some cold symptoms may persist for up to 14 days in one quarter of patients.
The first signs of a cold
A dry scratchy sore throat is often the first sign of a common cold and this may be due to the virus first infecting the back of the nose15.
Sneezing and a clear watery nasal fluid are also early signs of infection.
Is it a cold or flu?
Headache, fever and muscle aches and pains are commonly associated with influenza and bacterial infections but they are also associated with common cold viruses.
You cannot distinguish between a cold and flu!
You cannot separate a cold and flu just from the symptoms.
In general influenza has a sudden onset and is associated with fever and muscle aches and pains but a severe common cold can also cause these symptoms.
Fever is common in children
Fever associated with common cold is uncommon in the adult but quite common in infants and children.
How does the nose get blocked?
A blocked and stuffy nose is one of the most bothersome symptoms of common cold.
Nasal congestion is caused by the swelling of large veins lining the nasal passages. The degree of swelling of the nasal veins is controlled by nerves which release a chemical called norepinephrine or noradrenaline. Topical and oral nasal decongestants mimic the action of norepinephrine16.
Cough is usually a later symptom of colds and it is caused by inflammation and infection of the larynx, trachea and lower airways. The dry non-productive cough is related to inflammation of the upper airway and the productive chesty cough to inflammation of the lower airways.
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