PARIS : The influenza virus is a tiny, redoubtable foe that survives by stealth and sheer numbers.
Seen through an electron microscope, it resembles a spiky ball, comprising a protective shell studded with rods. It measures around 100 nanometres (100 billionths of a metre, 0.000004 of an inch) across, which is about a thousand times smaller than a bacterium.
Because it is so minute, the virus is unable to carry around the enzyme toolkit that it needs to reproduce. Instead, it hijacks the machinery of cells in the throat, nose and lungs to do this.
It first enters the nose or mouth, inhaled in droplets expelled by a cough or a sneeze by an infected person.
A virus can also survive on surfaces for up to 72 hours, depending on the type of surface, temperature and humidity.
This means the virus can picked up on the fingers and transferred to the mucous membranes if the person touches their nose, eyes or mouth.
The virus uses its spike to bind to, and then invade, an epithelial cell in the respiratory tract.
Once inside, it releases a package of genetic instructions, called RNA, that use the cell’s machinery to make parts for new virus particles.
The parts are knitted together to form hundreds of new virus particles that then burst out of the now wrecked cell and go on to infect other cells.
Cells that are attacked in the throat, lungs and muscles give rise to the classic symptoms of a sore throat, respiratory wheeze and muscle ache.
The high fever that is also a hallmark of flu is a response of the immune system against the invader. This defensive reaction continues until the viruses are eliminated.
Most people recover without complications after a week or two, but the disease can be dangerous for people with a chronic condition such as asthma or heart disease, or for the elderly, very young and others with a weaker immune system.
It can also lead to bacterial infection, such as bronchitis or life-threatening pneumonia.
Flu viruses fall into three main families.
Type A, the commonest, not only circulates among humans but also among birds and pigs, providing a unique opportunity to acquire new genetic variants that can leap the species barrier and spark a pandemic.
Type B can also cause epidemics, but usually produces a milder disease than Type A. Type C viruses, like Type B, are humans-only pathogens but have never been associated with a large epidemic.
Virus families are further sub-divided according to their two surface proteins, haemagglutinin (H) and neuraminidase (N).
Mexico’s swine flu virus has sparked alarm because it it presents a new genetic mix for which there may be no immunity and for which it will take months to devise and produce a vaccine.
It is a substrain of H1N1, presenting a new pattern of genes that resulted from a mix of existing viruses of human, pig and avian flu. Still unclear, though, is how contagious and virulent it is.