What separates a cold from a flu?

A look at the viruses that plague people in the winter months

Sniffy noses, coughs, sleepy eyes, fatigue, fever. This time of the year, many of us come down with a bug or flu or the cold. But the common illnesses that we get in these cold winter months are often mixed up – what exactly is it that makes us sick?

You would be forgiven for thinking that the flu and the common cold are the same infection separated only by severity of the disease. But although the two share some similar symptoms, they are actually caused by two entirely different viruses. The common cold is predominantly caused by the Rhinovirus, the most common viral infectious agent in humans. The average adult gets two to three colds every year. There are no vaccines or treatments for the common cold, as it is a mild illness that interrupts our lives only for a few days, even if it does make us feel a bit miserable. Common cold symptoms normally begin within 16 hours of exposure and peak within two to four days after onset. The symptoms are usually less severe than that of the flu, with fever, chills and severe headaches considered uncommon or rare. However, a stuffy nose, sore throat, and sneezing are characteristic of the infection.

“In the 20th century alone, three major deadly influenza outbreaks occurred.”

Influenza, or the flu, is an infectious disease caused by the Influenza virus and a much more serious infection. There are four types of the virus, three of which use humans as hosts. Influenza A is the most virulent of the four, and the most common type to infect humans, responsible for some deadly outbreaks in recent history. The virus manifests in fever, aches, chills, and fatigue. The flu kills more Americans annually than any other vaccine-preventable disease. Every year, Influenza spreads around the world in outbreaks, resulting in three to five million cases and about 250,000 to 500,000 deaths. Although we associate the disease with winter in the northern hemisphere, outbreaks can occur at any time in places around the equator. In the 20th century alone, three major deadly influenza outbreaks occurred. The Spanish influenza in 1918 led to approximately 50 million deaths, the Asian influenza in 1957 led to two million deaths, and the Hong Kong influenza in 1968 led to one million deaths.

“The virus is not what we traditionally understand as a “living thing” – it cannot reproduce on its own, and instead hijacks the host’s machinery in order to replicate.”

According to the Center for Disease Control, any person with the flu can transmit it to another person up to six feet away through respiratory droplets, released when an infected person coughs, sneezes, or talks. Each one of these droplets is filled with virions, which find their way inside an unfortunate victim. The surface of an influenza virus is covered with two viral proteins: hemagglutinin (HA) and neuraminidase (NA). Once inside the host, the virus uses these two proteins to get inside the host’s cell. The virus is not what we traditionally understand as a “living thing” – it cannot reproduce on its own, and instead hijacks the host’s machinery in order to replicate. When the virus releases its Ribonucleic Acid (RNA) inside the host cell, the RNA enters the cell’s nucleus and proteins are eventually made. These proteins re-assemble into a lot of progeny viruses, which escape the cell to invade other cells. For every one virus that enters a cell, millions can come out.

“Flu season” is what scientists have dubbed the cold half of the year, when incidences of flu and cold increase. Although the exact reason for the seasonal nature of flu outbreaks is unknown, there are a few proposed explanations. For one, cold temperatures allow for viruses to be preserved due to the fact that they can decompose slower. The result is that they linger for longer on exposed surfaces such as doorknobs and countertops. A 2007 study by researchers from New York showed that the transmission of the virus through aerosols is enhanced in cold and dry air. Viruses degrade faster in more humid air, and although hosts’ immune systems are not impaired by lower temperatures, they shed the virus for a longer period of time in the cold.

Cold temperatures also mean drier air, dehydrating mucous membranes which many consider to be the body’s first line of defence against pathogens. With the mucous membrane unable to function optimally, viruses such as Influenza are able to invade hosts more easily. Lastly, people tend to stay indoors more during winter, which increases person-to-person contact and promotes transmission of the virus.

Influenza viruses are constantly changing, which is why a different influenza vaccine must be administered every year. This is due to a phenomenon called “antigenic shift”, which is an abrupt, major change in the Influenza A virus. These proteins are recognised by a host’s immune system and are what is used as targets for antiviral drugs. However, due to DNA exchange, the HA and the NA particles in the Influenza A virus change every year, resulting in different serotypes of the virus. The most recent 2009 influenza pandemic, swine flu, was caused by the same H1N1 virus responsible for the Spanish influenza, but it had a new combination of genes from American pigs, Eurasian pigs, birds, and humans. The most prominent serotype in any year is the serotype for which a vaccine is prepared.

“The vaccine is a cocktail of harmless viruses which preps our immune system, so that when the “real” influenza virus attempts to invade our cells, our immune system is ready to fight.”

The best way to evade a flu infection is by getting an Influenza vaccine. The vaccine is a cocktail of harmless viruses which preps our immune system, so that when the “real” influenza virus attempts to invade our cells, our immune system is ready to fight. Usually, three or four strains of viruses are included in the vaccine, strains that are determined by experts to be the ones most likely to circulate in any given year. If these experts’ forecasts are correct, then the vaccine offers good protection as they will match the year’s circulating viruses. Getting a vaccination reduces infection risk by 60%, and in a bad year reduces the risk by 20-30%.

Although viruses can be seen as little monsters that effortlessly invade and hijack our bodies, you should remember that our immune systems are highly competent machines which can take care of those pesky virions most of the time. However, we must do as much as we can to help it. From simply washing hands to getting a vaccine, there are plenty of things that we can do to avoid getting these pesky winter diseases.

Danielle Olavario

Danielle Olavario

Danielle Olavario is the current SciTech Editor of Trinity News. She is a Senior Sophister Microbiology student.