The common cold, also called rhinitis, is a viral infection of the upper respiratory tract, which includes the linings of the sinuses (cavities in the head behind the nose and eyes), throat, and pharynx. The word “rhinitis” means inflammation of the nose. The common cold is indeed common, being the infectious disease most often caught by human beings. Cold symptoms include runny nose, sore throat, tiredness, and sometimes coughing or sneezing. The common cold is never fatal in people with normal immune systems. The viruses that cause colds exist in a great variety of slightly different forms, and although a person cannot catch the same cold—that is, be re-infected by exactly the same cold virus—twice, there are always plenty of other colds waiting to be caught. There is no vaccine for the common cold for the same reason. Because a cold is a viral infection, anti-biotics do not affect it.
Most colds are caused by an adenovirus or a coronavirus. The only other animals that can be infected by these viruses are the few primates most closely related to humans, including chimpanzees. Colds have been known throughout recorded history. Over two thousand years ago, the Egyptians represented a cold by a drawing of a nose followed by a symbol for something coming out. Pre-modern European doctors thought that colds were caused by an imbalance of the four “humors” (blood, yellow bile, black bile, and phlegm). The existence of disease-causing microorganisms was not known until the 1800s, and viruses were not known until the 1890s.
Even after the discovery of viruses, doctors mistakenly thought that colds were caused by bacteria for many years. The viral nature of colds was discovered in the early twentieth century, when Walter Kruse, a German researcher, showed that colds could be transmitted by nose secretions passed through a filter having holes too small for bacteria to pass. The actual viruses causing most colds were isolated and grown in culture in laboratories in the 1950s and 1960s.
Cold symptoms include stuffy nose, runny nose, mild fever and chills, tiredness, sore throat, cough, impairment of smell and taste, and hoarseness of voice. The average duration of a cold is 7.4 days; mild colds last only two to three days and about 25% of colds last about two weeks. Symptoms are not caused directly by the virus interfering with body functions, but by the body's defensive response to the virus. When cells in the respiratory tract are infected, substances called inflammatory mediators are released by the body. These cause small blood vessels to widen, which makes tissue swell. They also increase mucus secretion, stimulate pain-sensing nerve fibers, and activate cough and sneeze reflexes. The body eventually clears itself of a cold by learning to identify specific molecules, called antigens, which exist only on the surface of the particular cold virus causing that cold. Immune-system cells can then attack anything in the body that bears these antigens. The antigens on each cold virus are slightly different, which is why the body has to learn from scratch how to fight every new cold.
About 1–5% of colds are complicated by acute bacterial sinusitis, a bacterial infection of the sinuses that can have serious side effects, including eye infection and meningitis. Unlike cold viruses, the bacteria that cause bacterial sinusitis can be killed using antibiotics.
Mild cases of influenza (also called flu) resemble colds; more severe cases cause the usual cold symptoms but also muscle aches, fever, and a more severe cough. However, influenza is a distinct disease from the common cold.
Colds are usually contracted when cold-virus particles are picked up by touching a person with a cold or a surface contaminated with the cold virus. Cold-virus particles are then often transferred to the nostrils or eyes, again by touch. A virus can be inhaled into the nostrils and deposited in the back of the adenoid area (behind the soft palate at the back of the mouth). Virus particles in the eyes are transported down into the nasal passages and then to the adenoid area. There they colonize cells, which is why many colds begin with a sore throat. Some colds may be transmitted by airborne mucus particles ejected by sneezing. As few as one to 30 virus particles introduced into the nose can reliably produce an infection.
Cold viruses colonize cells by attaching to a molecular structure on the surface of the cell called a receptor (specifically, the ICAM-1 receptor). After attachment, the virus is absorbed into the cell, where it tricks the cell into manufacturing more of the virus. Eventually the cell produces so much virus that it ruptures, releasing many new virus particles. The cycle of virus reproduction takes about eight to 12 hours. Cold symptoms begin about 10 hours after infection and symptoms peak between 36 and 72 hours after infection.
Colds afflict all nations, climates, and social classes about equally. Typically adults suffer one to four colds per year and children suffer six to ten colds annually.
Despite a widespread assumption that exposure to cold temperatures causes colds, populations living in colder climates do not get more colds. A 2005 experiment counting cold symptoms reported by groups who either underwent controlled chilling of the feet or did not showed a higher rate of symptoms among subjects whose feet had been chilled. However, the study has been criticized for not verifying whether experimental subjects who reported symptoms actually had colds. There is no scientific consensus that being chilled increases one's chance of catching a cold.
There is no effective treatment for the common cold. Contradictory evidence exists for the effectiveness of herbal treatments, zinc gluconate, and vitamin C, but there is no scientific agreement that any of these substances decrease one's chances of catching a cold, the length of a cold, or the severity of a cold. Cold treatment primarily targets the symptoms of the infection. Cold medicines often include antihistamines to reduce mucus production, pain relievers, cough suppressants, and alcohol and other drugs to induce sounder sleep.
Experimental antiviral drugs have shown some ability to combat the common cold, but scientists question whether the use of these drugs is appropriate, given the harmlessness of the common cold and the high cost and possible risks of antiviral drugs.
Colds can be prevented by following good hygiene practices. Four basic steps are recommended by diseasetransmission specialists: washing hands, avoiding close contact with persons who have colds (or, if you have a cold, avoiding close contact with uninfected persons), covering up when sneezing or nose-blowing, and, for health-care professionals, wearing masks and clean gloves.
WORDS TO KNOW
ANTIGEN: Antigens, which are usually proteins or polysaccharides, stimulate the immune system to produce antibodies. The antibodies inactivate the antigen and help to remove it from the body. While antigens can be the source of infections from pathogenic bacteria and viruses, organic molecules detrimental to the body from internal or environmental sources also act as antigens. Genetic engineering and the use of various mutational mechanisms allow the construction of a vast array of anti-bodies (each with a unique genetic sequence).
COLONIZATION: Colonization is the process of occupation and increase in number of microorganisms at a specific site.
RHINITIS: An inflammation of the mucous lining of the nose, rhinitis is a nonspecific term that covers infections, allergies, and other disorders whose common feature is the location of their symptoms. These symptoms include infected or irritated mucous membranes, producing a discharge, congestion, and swelling of the tissues of the nasal passages. The most widespread form of infectious rhinitis is the common cold.
There are at least 500 million colds per year in the U.S. population of about 300 million people, causing about 20 million lost workdays for adults and 21 million lost school days for children. The direct costs of colds, including purchase of cold remedies, are $17 billion per year in the U.S.; indirect costs, including lost productivity, are $22.5 billion. Lost workdays are a far more significant hardship for persons in nonprofessional, unskilled, or service-sector jobs that entitle the worker to few or no paid sick days.
In 2006, research at the Mayo Clinic indicated that some viruses that cause colds—picornaviruses—may damage the brain, causing cumulative loss of memory over a lifetime. “Our findings suggest that picornavirus infections throughout the lifetime of an individual may chip away at the cognitive [thinking ability] reserve, increasing the likelihood of detectable cognitive impairments as the individual ages,” the researchers reported. There is no proof that picornavirus-caused colds do cause memory loss in human beings, but this is an active area of research.
IN CONTEXT: MANY VIRUSES CAN CAUSE COLDS
There are over 200 viruses that can cause colds. Most are rhinoviruses and coronaviruses, although several other types are known. The viruses that cause colds are RNA viruses; that is, they contain RNA (ribonucleic acid), a type of long, ribbonlike molecule that encodes information. RNA viruses reproduce by tricking body cells into producing proteins and RNA according to the instructions in the viral RNA. These proteins and pieces of viral RNA then self-assemble into new virus particles.
IN CONTEXT: REAL-WORLD RISKS
The National Institute of Allergy and Infectious Diseases (NIAID) asserts that research data does not support the popular linkage of colds to cold weather—or the development of the common cold from a person becoming either chilled or overheated. NIAID asserts that data developed by researchers find that “these conditions have little or no effect on the development or severity of a cold. Nor is susceptibility apparently related to factors such as exercise, diet, or enlarged tonsils or adenoids. On the other hand, research suggests that psychological stress, allergic disorders affecting the nasal passages or pharynx (throat), and menstrual cycles may have an impact on a person's susceptibility to colds.”
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