Influenza
Influenza (or as it is commonly known, the flu or the grippe) is a contagious disease, caused by an RNA virus of the orthomyxoviridae family. It rapidly spreads around the world in seasonal epidemics. The name comes from the old medical belief that unfavourable astrological influences cause the disease.

Types:
There are three types of the virus, identified by antigenic differences in their nucleoprotein and matrix protein:

Influenza A viruses that infect mammals and birds Influenza B viruses that infect only humans Influenza C viruses that infect only humans The A type of influenza virus is the type most likely to cause epidemics and pandemics. This is because the influenza A virus can undergo antigenic shift and present a new, immune target to susceptible people. Populations tend to have more resistance to influenza B and C, because they only undergo antigenic drift, and have more similarity with previous strains.

The term superflu is used to refer to a strain of flu that spreads unusually quickly, is unusually virulent, or is for which the host is uncommonly unresponsive to treatment. Thus, there is a tendency to apply the term to strains which cause epidemics or pandemics. There is no exact scientific definition of a superflu.

Symptoms :
The virus attacks the respiratory tract, is transmitted from person to person by saliva droplets expelled by coughing, and causes the following symptoms:

Fever Headache Tiredness (can be extreme) Dry cough Sore throat Nasal congestion Sneezing Irritated eyes Body aches Extreme coldness Fatigue

Flu season
Influenza reaches peak prevalence in winter, and because the Northern and Southern Hemisphere have winter at different times of the year, there are actually two flu seasons each year. Hope-Simpson (1981) observed that influenza outbreaks are globally ubiquitous, and consistently occur six months following the time of maximum solar radiation in an area. Therefore, the World Health Organization makes two vaccine formulations every year; one for the Northern, and one for the Southern Hemisphere.

While most influenza outbreaks in the Northern Hemisphere tend to peak in January or February, not all do. For example, the influenza pandemic of 1918 and 1919 reached peak virulence during late spring and summer worldwide, and not until October in the US. It remains unclear why outbreaks of the flu occur seasonally rather than uniformly throughout the year. One possible explanation is that, because people are indoors more often during the winter, they are in close contact more often, and this is enough to trigger the outbreak. Another is that the cold weakens the immune system; however, the virus is contracted in a warm indoor environment in which it can thrive.

Common cold
The common cold is a mild viral infectious disease of the nose and throat; the upper respiratory system. Symptoms include sneezing, sniffling, running/blocked nose (often these occur simultaneously, or one in each nostril); scratchy, sore, or phlegmy throat; coughing; headache; and tiredness. Colds typically last between three to five days, with residual coughing lasting up to three weeks. As its name implies, it is the most common of all human diseases, infecting subjects at an average rate of slightly over one infection per year per person. Infection rates greater than three infections per year per person are not uncommon in some populations. Children and their caretakers are at a higher risk, probably due to the high population density of schools and the fact that transmission to family members or caretakers is highly efficient.

Ninety-five percent of people exposed to a cold virus become infected, although only 75% show symptoms. The symptoms start 1-2 days after infection. They are a result of the body's defense mechanisms: sneezes, runny nose and coughs to expel the invader, and inflammation to attract and activate immune cells. The virus takes advantage of sneezes and coughs to infect the next person before it is killed by the body's immune system. Sneezes expel a significantly larger concentration of virus "cloud" than coughing. The "cloud" is partly invisible and falls at a rate slow enough to last for hours - with part of the water droplets evaporating and leaving much smaller and invisible "droplet nuclei" in the air. Droplets from turbulent sneezing or coughing or hand contact can also last for hours on surfaces, although less virus can be recovered from porous surfaces such as wood or paper towel than non-porous surfaces such as a metal bar. After a common cold, a sufferer develops immunity to the particular virus encountered. However, because of the large number of different cold viruses, this immunity is of limited use. A person can therefore easily be infected by another cold virus to start the process all over again.

Prevention It is possible to get vaccinated against influenza. However, due to the high mutability of the virus, a particular flu vaccine formulation usually only works for about a year. The World Health Organization co-ordinates the contents of the vaccine each year, to contain the most likely strains of the virus which probably will attack the next year. The flu vaccine is usually recommended for anyone in a high-risk group, who would be likely to suffer complications from influenza.

The best way to avoid a cold is to avoid close contact with existing sufferers, to thoroughly wash hands regularly, and to avoid touching the face. Anti-bacterial soaps have no effect on the cold virus - it is the mechanical action of hand washing that removes the virus particles. In some countries, such as China and Japan, people with the common cold wear surgical masks out of courtesy to protect others.

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