WASHINGTON – Here are some questions and answers about the science of swine flu – the H1N1 virus that’s sweeping the world:

Q: What exactly is a virus?

A: It’s a tiny packet of only eight genes wrapped in a cloak of proteins, much smaller than a bacterium. Unlike bacteria, a virus is only half alive. It can’t eat or reproduce on its own, but must take over the genetic machinery of a living cell. Most viruses are harmless; some are useful, but others, such as the flu virus, can be deadly.

Q: What makes this swine flu virus special?

A: It’s a novel combination of bird, pig and human viral genes never before found in the U.S. or elsewhere, so people have no immunity to it. It’s a descendant of the H1N1 virus that killed tens of millions of people worldwide in the pandemic of 1918-1919, mixed in with recent strains of swine and bird flu viruses. The 1918 virus originated in birds and then jumped to humans. This year’s virus apparently jumped from a pig to a 5-year-old boy in Veracruz, Mexico, who passed it on to other humans.

Q: What does H1N1 stand for?

A: It’s the initials of two sugar proteins (their scientific names are hemagglutinin and neuramidinase) that sit on the surface of the virus and do its dirty work. There are 16 types of the H protein, numbered H1 through H16, and 9 types of the N protein, numbered N1 through N9. That makes 144 possible combinations of the virus, a constantly changing challenge for prevention or treatment. A new combination, H2N2, cause a brief swine flu epidemic in 1957. An H3N2 strain was the source of another epidemic in 1968. The bird flu virus that began in Southeast Asia a decade ago and has spread throughout the Old World is an H5N1 combination.

Q: How does the H protein work?

A: The H protein looks like a little spike that fits into a notch, called a receptor, on the outside of an animal or human cell and lets the virus enter. Once inside, the virus hijacks the DNA in the cell’s nucleus and uses it to make copies of itself.

Q: What does the N protein do?

A: After infection, it opens a passage in the cell wall and releases the new baby viruses, which can now invade other cells. Without the N protein, infection would be limited to the first cell, rarely enough to cause disease.

Q: How do medicines such as Tamiflu and Relenza work?

A: They block the action of the N protein so the virus can’t spread. They’re not vaccines to prevent an infection, but drugs to limit its impact. They should be taken as soon as possible since the virus reproduces most rapidly between 24 and 72 hours after illness begins.

Q: How does a new virus develop?

A: When the genes that govern the H and N proteins reproduce, random changes – mutations – can occur in their DNA. The changes gradually accumulate, ultimately producing a virus that may be more lethal or may penetrate a target cell more easily. Another possibility is gene-swapping. This can happen when a cell is infected by viruses from different creatures, say a chicken and a pig. The cell becomes a “mixing bowl,” whipping up a new virus containing some chicken genes and some pig genes. In the new strain of H1N1 virus, pieces of human, bird and pig genes are all scrambled up.

Q: How does this H1N1 virus differ from the H1N1 that caused the Spanish flu pandemic in 1918-1919?

A: That virus developed various changes over the years, so it’s similar but not identical to its ancestor – like a grandson who resembles but also differs from his grandfather. So far, H1N1 is not as virulent as the previous strain, but that could change. The earlier pandemic began mildly in 1918, but returned in a devastating second wave six months later. Experts fear that could happen again. Hence they are rushing to develop a vaccine by this fall.

Q: How does H1N1 virus jump from animals to humans?

A: Usually the H protein on a pig or bird virus doesn’t fit easily into the receptor of a human cell. So a person exposed to such a virus is unlikely to get infected. However, random changes may occur in the genes that control the shape of the H protein and allow the virus to pass through the cell wall. This gives rise to a new strain of H1N1 that’s adapted to humans. Now the virus can pass from person to person, as is happening now. Contact with infected pigs or birds no longer is necessary.

Q: How does one person catch H1N1 from others?

A: The virus can be transmitted through the air – by a cough or a sneeze – or by a handshake or by touching an infected surface, such as a doorknob. The virus can live for up to two hours outside a cell. It can’t be passed by eating pork.

Q: Why does this disease seem to be more deadly in Mexico than in the U.S. or other countries?

A: That may be an illusion. The first U.S. death has now occurred in Texas and more fatalities are expected. It’s possible that many cases of mild disease in Mexico went undetected, making the mortality rate appear to be higher there than it does here. It’s also possible that the virus strain in the U.S. differs slightly from the one in Mexico, making it less virulent.

Q: Why does H1N1 seem to attack healthy young adults more than sick and elderly people, who are most affected by the ordinary seasonal flu?

A: Young adults have a healthy immune system that launches a massive counterattack of antibodies against the flu virus. Unfortunately, the counterattack can cause an overwhelming inflammation that damages other organs, such as the lungs. Elderly people with weaker immune systems are less likely to suffer from such harmful inflammation. Older people who were exposed to earlier flu epidemics might

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