What is Cold?
A viral infection involving the
upper respiratory tract is called cold. It is also called rhinopharyngitis,
nasopharyngitis and acute coryza. There are over 200 viruses which cause cold. Most
of these viruses are rhinoviruses. The symptoms of cold are cough, sore throat,
blockage and runny nose, sneezing and fever. Cold is one of the commonest
illnesses humans face each year. Everyone gets at lest one attack of cold each
year. The symptoms of cold may last seven to ten days. Even though it is very common, the attacks of
cold are self limiting and therefore, does very little harm compared to flu.
What is Flu?
Flu, also known as
influenza is an illness caused by influenza viruses. Influenza viruses are a
family of RNA viruses called orthomyxoviridae. The symptoms of flu include chills,
fever, sore throats, myalgia (muscle pains), severe headache, fatigue, and generalized
feeling of ill health. The word ‘influenza’
may remind you of the recent outbreaks of bird flue and swine flu, which became
pandemics.
Cold and Flu Symptoms
Even though the symptoms of flu
seem similar to those of cold, flu makes the affected individual severely
ill. The symptoms of cold and flu occur
as a result of the immune response against the invading virus. Viruses causing
cold and flu enter the body mainly through the air. Some viruses, like
rhinoviruses enter the body by direct contact. Once they are entered to the
body, they bind to the specific receptors in the cell membranes of the
respiratory tract. The viruses causing
cold and mild flu can only bind to the cells in the upper respiratory tract.
But, some viruses such as HINI virus bind to lower respiratory tract and
therefore, cause more severe diseases such as pneumonia.
After binding to the cell
membrane, the virus enter into the cell and start replicating and producing
molecules they require. The host cell identifies these viruses as foreign
bodies and release inflammatory mediators (cytokines and chemokines). These
mediators goes and act on other cells and start an inflammatory process which
finally gives rise to the symptoms.
1) Sneezing and Runny Nose
The inflammatory mediators start
an inflammatory process within the respiratory epithelium. As a result the
edema develops and the cells start secreting mucus. The produced mucus comes
out of the nostril. That is what is experienced as runny nose. It also causes irritation
of the nerves which starts the sneezing reflex. The impulses are carried to the
sneezing center of the brain, which commands the chest muscles, the diaphragm,
the abdominal muscles and the muscles of the larynx to contract. This
ultimately results in a violent blowing of air out of the nose. The high
velocity of the air carries some of the mucus out along with the viruses. The
process finally helps to quicken the healing process by reducing the viral
load. But, it also creates a route for the virus to find another host.
The inflammatory mediators
released by the cells go and act on the hypothalamus. Hypothalamus is the
centre for thermoregulation. These inflammatory mediators increase the
temperature set point. So, the
hypothalamus sends signals to the body to produce more heat. Heat is produced
by contraction of the muscles. This finally causes fever. In the same way the
patient has chills when the hypothalamus is set to a lower temperature.
3) Sore throat
Viral infection itself and breathing
through the mouth damage the epithelium of the throat. This starts an
inflammatory process in the throat. The patient feels this as sore throat.
4) Cough
As mentioned above, the infection
of the respiratory tract starts an inflammatory process. One of the components
of the inflammatory process is excess mucus secretion. This excess mucus is
passed in to the throat. The mucus irritates the throat mucosa. This triggers
the cough reflex. The violent coughing helps to bring the excess mucus out from
the lower respiratory tract and expel them out.
Lymph nodes are the places where
most of the lymphocytes lie. These lymphocytes proliferate and produce
antibodies to fight against the invading organisms. This process causes the
lymphoid tissues to swell.
6) Generalizes feeling of ill
health
The inflammatory mediators
released during an attack of flu or cold act on almost all the cells in the
body. They alter the normal cellular functions. It finally results in
generalized body weakness.
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