We catch colds twice a year, and the viruses we infect are different every 100 years丨Virus Super Topic

Cold is a general term for a class of diseases with similar symptoms. The cause is an upper respiratory tract infection caused by a virus. There are more than 200 viruses that can cause colds. The harmfulness of a virus is closely related to its prevalence, pathogenicity and lethality. Some viruses can cause severe lower respiratory tract infections or pandemics. Colds should not be taken lightly.

Written by Li Qingchao (Shandong Normal University)

Fever, fatigue, sneezing; headache, sore throat, runny nose. You have a cold! That’s right, the change of seasons and the cooler weather are the best times for the “cold virus” to wreak havoc. In autumn and winter, when people encounter these symptoms, nine out of ten times they can make a “correct diagnosis”. After all, colds are too common for each of us. On average, adults catch colds two to three times a year, while children may catch them six to eight times. The course of the illness can be as short as seven days or as long as three weeks[1].

Everyone has had a cold, and everyone knows what it feels like, but what causes a cold? The answer may be more complicated than you think - there are quite a few "cold viruses".

A cold is not a disease, but a class of diseases

As early as 1500 BC, the ancient Egyptians recorded the symptoms of colds on the Ebers papyrus. Chinese medicine calls it "shangfeng", which means "a mild external infection caused by wind evil invading the surface", which was mentioned in "Shanghan Zhige" around 1200 AD; starting from the 16th century, English called it "cold", and the common cold was called "common cold".

However, the term "cold" appeared relatively late. According to investigations, the formation of this term is related to the reasons for officials to register for leave. From the emergence of the term "感风" (feeling wind evil) in the Song Dynasty, to "冒风" (symptoms "burst" after being exposed to wind), and then to the Qing Dynasty, "冷寒" became the most common excuse for officials to take leave. Later, the term "冷寒" was gradually accepted by the people.

"Cold" is not so much a disease as a general term for diseases with similar symptoms. Its essence is an acute upper respiratory tract infection . So, when someone asks you "what's wrong with you", you say "I have a cold", what you are actually saying is: "I have an itchy throat, sneezing, coughing, runny nose, stuffy nose, sore throat, headache, fever..." Of course, there are also serious colds that can develop into lower respiratory tract infections or even pneumonia.

So how do we catch a cold? Ancient Chinese and foreigners have a consensus on this question - in English, we say "I caught a cold", and ordinary people often say "get a cold", which basically means the same thing, that temperature changes cause colds. Of course, whether it is wind, coolness or cold, they are not the cause of colds - viruses are the real cause of colds.

Viruses that cause colds

In fact, the name "cold virus" can be said to be "quite disrespectful" to the virus. The so-called "cold virus" is not a strict classification definition. Many viruses are just lumped together and called "cold virus" because they all infect the upper respiratory tract.

In fact, there are more than 200 viruses that can infect the human body and cause colds. Usually, a cold patient may have one or more viruses in his body. Since most colds themselves are not serious, the culprits that cause colds are not that important. Therefore, most cold viruses are not well-known to the public and can be said to be "passers-by and invisible people who do not deserve to have a name."

There are no cold viruses in the world, only viruses that cause colds. These viruses are varied and numerous. If you catch a cold twice a year, and are infected with a different virus each time, you will never be infected with all of them even if you live a hundred years. Of course, it is common that some viruses infect us repeatedly throughout our lives, while some viruses have little "presence". Let's take a look at these so-called cold viruses from the perspective of virus classification, in order of the number of cold cases they cause.

1. Rhinovirus

In 1914, Walter Kruse, a hygienist at the University of Leipzig, proposed that colds are caused by viral infections. British and American scientists continued to study this during World War I and World War II, and established the Common Cold Research Institute in 1946. In 1956, they finally discovered that rhinovirus is one of the real causes of colds. It is also the most common cold virus, causing 30%-80% of colds.

Rhinovirus belongs to the Picornaviridae family and the Enterovirus genus. It is divided into three types: A, B, and C. More than 160 types are currently known. Rhinovirus is a non-enveloped single-stranded positive-strand RNA virus with a particle diameter of 30nm and a genome length of about 8 kilobases. Both the particle and the genome are relatively small among viruses.

Rhinovirus

Rhinovirus can be transmitted through droplets or contact. It can infect the human nasal cavity within 15 minutes after entering the nasal cavity. The most suitable temperature for survival is 33-35°C. The incubation period is about 20 hours to 4 days. Within 2 days of infection, more than 50% of people will have symptoms. Symptoms of rhinovirus infection include sore throat, runny nose, nasal congestion, sneezing and coughing; sometimes accompanied by muscle aches, fatigue, malaise, headache, muscle weakness or loss of appetite. The cold caused by rhinovirus is a typical self-limiting viral disease process that can heal on its own.

2. Coronavirus

About 15% of common colds are caused by coronaviruses. Among them, human coronaviruses HCoV-OC43, HCoV-HKU1, HCoV-229E, and HCoV-NL63 continue to circulate among humans and cause mild symptoms of the common cold in adults and children worldwide [2].

Of course, there are far more coronaviruses that can cause human diseases than the ones mentioned above. Coronaviruses are a class of enveloped single-stranded positive-strand RNA viruses with a large genome of about 30,000 bases, which can cause various diseases in humans or animals. The coronavirus family has produced several tough characters, such as the atypical pneumonia virus and the Middle East respiratory syndrome virus. The new coronavirus has caused great harm to humans in the past three years.

3. Influenza viruses

Influenza virus belongs to the family of Orthomyxoviridae and can be divided into four types: A, B, C, and D (respectively called A, B, C, and D in Chinese). Each virus is divided into different subtypes according to different serotypes. Among them, the influenza A H1N1 subtype that broke out in 1918 caused the deaths of about 50 million people in the world (see "Decoding the 1918 Flu: The Battle for Human Survival Will Not End in a Hundred Years | Unfolding"). Highly pathogenic influenza viruses such as H2N2, H3N2, and H5N1 subtypes all belong to influenza A (A). When people mention influenza (influenza), they generally refer to the severe cold caused by these few highly pathogenic influenza viruses. It is not the common cold that people usually call it. Influenza is much more serious to human health than the common cold.

However, not all influenza viruses cause influenza. About 10% to 15% of common colds are actually caused by influenza viruses. In addition to the influenza viruses mentioned above, most serotypes of influenza A viruses are not extremely pathogenic, and their symptoms are difficult to distinguish from the common cold. They can only cause the common cold: although they are infected with influenza viruses, the symptoms show that they are just colds.

In a nutshell, influenza virus is a type of virus, but not all influenza viruses cause severe viral influenza. More influenza viruses only cause the common cold.

4. Adenovirus and other viruses that can cause colds

Adenoviruses are a class of non-enveloped DNA viruses. Some adenoviruses can cause mild infections of the upper respiratory tract, causing symptoms of the common cold.

Human respiratory syncytial virus (HRSV) is a virus that causes respiratory tract infections. Infected mucosal cells can fuse together to form syncytia. Infecting adults, HRSV causes only mild cold symptoms, but it is a major cause of lower respiratory tract infections and hospital visits in infancy and childhood [3].

Human parainfluenza virus (HPIV), a member of the Paramyxoviridae family, is the second leading cause of hospital admission for respiratory illness in children under 5 years of age [4].

Human metapneumovirus (HMPV) is a negative-sense single-stranded RNA virus of the Pneumoviridae family. The clinical features and severity of HMPV are similar to those of HRSV. HMPV is also an important cause of disease in the elderly.

These viruses specifically target children and the elderly, which is despicable, but they are also very common in the virus world.

Why do viruses always attack the respiratory tract?

Ever since our ancestors crawled out of the sea, they have been faced with a problem: breathing. As the saying goes, water is the source of life. When we breathe air, the gas exchange process generally needs to be carried out on the moist surface of cells. For the same reason, viruses cannot infect cells in dry conditions. They need to enter the human respiratory tract through airflow, and the upper respiratory tract has therefore become the most susceptible part of the human body to infection.

In order to produce clean, moist and temperature-appropriate air to protect the lungs, the human body uses nasal hair to filter the air, the nasal mucosa moistens the air and absorbs dust, and the cilia on the surface of the tracheal mucosa move to clean up foreign matter... When a large amount of air containing foreign matter rushes into the respiratory tract, it can cause coughing, sneezing and other activities, expelling foreign matter from the body along with nasal mucus or sputum, thus playing a role in cleaning the respiratory tract. It can be said that the human body has spared no effort to defend against viruses.

Despite this, the excellent "cold virus" can still sneak into the respiratory tract and infect the upper respiratory tract mucosa. When humans talk, cough, or sneeze, they will spray out saliva and other liquids containing the virus. The droplets containing the virus will float around the patient and after being inhaled into the upper respiratory tract by susceptible people, the virus will set up camp in the new upper respiratory tract of the human body. In addition, after the hands touch the surface of an object contaminated with the virus, and then habitually touch the face and rub the nose, the consequences are the same. Therefore, an important means of preventing respiratory viruses is to "wear a mask and wash hands frequently." It should be noted that people who have caught a cold need to actively wear a mask to avoid infecting their family members or other people. Susceptible people can also wear masks to prevent infection during the epidemic season.

There are big monsters among the minions

With the discovery of coronaviruses that can cause serious diseases in humans, especially the pandemic caused by the new coronavirus, the study of coronaviruses has received more and more attention. Since coronaviruses can infect a variety of animals, there are coronavirus reservoirs in nature, such as bats, which may also contain other strains that infect humans and cause pandemics. Therefore, the possibility of a new coronavirus pandemic in the future cannot be ruled out .

Influenza has experienced several pandemics in history, causing tens of millions of deaths. In fact, human health may always be shrouded in the shadow of influenza pandemics. This is because the host range of influenza viruses is very wide, especially the avian influenza virus reservoir in migratory birds, which hovers over humans and migrates periodically. During the migration of migratory birds, they can come into contact with local wild birds and poultry, introduce avian influenza to the local area, and then spread it to pigs. Human influenza viruses can also infect pigs. With pigs as intermediate hosts, influenza viruses from various sources co-infect the same pig, causing mixed infections. The influenza virus undergoes gene rearrangement and mutation in the pig, which may produce influenza mutants that are easily infected with humans. This virus host migration and mutation network of migratory birds → poultry → pigs → and then to humans makes it impossible to eliminate influenza, leaving human health exposed to the threat of influenza pandemics at any time.

Some people may say that for a minor cold, how can we put the 1918 influenza pandemic and the new coronavirus, which killed 50 million people, in our family tree to show off? This guy is mistaken. In fact, the family tree of viruses is not arranged according to the site of infection. As mentioned earlier, the term "cold virus" does not exist in the scientific community. In fact, "cold" is also a very bad concept to some extent, because it sums up the symptoms of upper respiratory tract infection into a seemingly extremely common and not-so-serious disease, covering up the fact that the pathogens that cause colds, that is, upper respiratory tract infections, are complex and diverse . The harmfulness of these pathogens varies greatly. Some cause common colds that cause sneezing and coughing, while others cause severe viral pneumonia that requires intubation treatment; the harmfulness of the same virus to people of different ages is also far apart: adults may only have mild symptoms or no symptoms, while infants, children, or the elderly face the risk of death.

The common cold itself does not exist obediently in the definition or imagination of humans. It may develop into a more serious lower respiratory tract infection at any time . Whether in clinical or scientific research, the "popularity" of a particular virus is closely related to its prevalence, pathogenicity and lethality. H1N1 (caused the 2010 influenza epidemic), SARS-CoV-1 (caused the 2003 SARS epidemic), MERS-CoV (caused the Middle East respiratory syndrome) and SARS-CoV-2 (caused the new crown epidemic since 2020), more and more respiratory tract infection viruses have been discovered and studied, standing out from the "cold", "gaining a name", and receiving people's attention. Take SARS-CoV-2 as an example. Its risk of developing into a serious lung infection is greater than other coronaviruses, but there are also a large number of mild or even asymptomatic patients. These patients are often ignored and treated as colds without being tested for the new coronavirus.

It is best not to take colds lightly. Even relatively weak viruses can cause trouble once they invade the lungs. High fever, persistent coughing, chest tightness and pain, or cold symptoms in infants, young children, or the elderly should all be closely monitored and treated promptly if necessary. In addition, secondary bacterial infections caused by viral pneumonia can also make the condition worse and prolong the course of the disease.

We can boldly assert that the most likely, most contagious and harmful virus in the next new viral pandemic will still be a respiratory virus that can be transmitted through the air, and it may just be a close relative of many "cold viruses" . Personally, it is better to strengthen your physical fitness and pay attention to protection. If you have a cold, you should pay attention to rest, pay close attention to your condition, and get well soon.

References

[1] Arroll, B (March 2011). "Common cold". Clinical Evidence. 2011 (3): 1510.

[2] Corman VM, Muth D, Niemeyer D, Drosten C (2018). "Hosts and Sources of Endemic Human Coronaviruses" . Advances in Virus Research. 100: 163–188.

[3] "Respiratory Syncytial Virus" . Center for Disease Control, Respiratory and Enteric Viruses Branch. October 17, 2008. RetrievedOctober 2, 2009.

[4] Schmidt, Alexander; Anne Schaap-Nutt; Emmalene J Bartlett; Henrick Schomacker; Jim Boonyaratanakornkit; Ruth A Karron; Peter L Collins (1 February 2011). "Progress in the development of human parainfluenza virus vaccines" . Expert Review of Respiratory Medicine. 5 (4): 515–526.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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