What is a virus?

A virus is a parasite made up of a shell of proteins and other organic compounds surrounding a small amount of genetic material, such as DNA.

Because they're made of the same basic stuff as plants and animals, it's tempting to conclude that a virus is alive. But most biologists don't consider viruses to be alive because they can't do something living things can: reproduce on their own. Viruses require the cellular machinery of a living host to make more viruses.

How does a virus infect a cell?

Depending on the type of virus, either the entire virus enters the cell through its membrane, or the virus sits on the membrane and injects its genetic material through it. Either way, the virus's genetic material, its instructions for making more viruses, gets inside the cell.

Once infected, the cell's chemical machinery, which would normally produce proteins based on its own DNA, is hijacked by the virus and produces nothing but parts for more viruses.

These parts can assemble themselves into complete viruses and quickly fill the cell. The new viruses then leave the cell, either a few at a time or all at once when the cell membrane breaks, killing the cell.

How do antivirals work?

Antibiotics, like penicillin, are only effective against certain types of bacteria and don't work on viruses. Antiseptics and vaccines are useful for preventing viral infections, but can't help once a virus has taken hold.

Antiviral drugs are a fairly recent development. In the 1980s, the full genetic sequences of certain viruses were found and scientists began to understand in detail how they work. Certain proteins were identified as essential to a virus's function and chemicals could be synthesized to block those chemicals.

Researchers began to search for proteins that were common to several types of viruses – to make an antiviral drug that was as broad as possible – but that were unique to viruses only, and didn't occur in people, to minimize the side-effects of the drug.

Drugs have been developed that block chemicals involved in every step of a virus's infection of a cell.

Some drugs block a virus's ability to enter a cell in the first place by blocking receptor proteins on a cell's surface. Two influenza drugs, amantadine and rimantadine, use this strategy.

Other drugs block a virus's ability to hijack the cell's machinery to produce more virus components. An antiviral used in cases of herpes, called acyclovir, uses this mechanism, as does zidovudine or AZT, the first antiviral drug approved to treat AIDS.

Another type of antiviral drug blocks the last stage of a virus's reproductive cycle, the release of newly created viruses from an infected cell. Two drugs used to treat the flu, zanamivir and oseltamivir, also known as Relenza and Tamiflu, use the strategy.

Another class of antiviral drugs uses a different tactic: boost the patient's immune system, rather than attacking the virus directly. Interferons are a naturally occurring protein produced as part of the immune system. Synthetic interferons have been used since the 1990s as a drug to treat viral infections, especially hepatitis.

How do viruses suddenly become more dangerous? Can this happen to avian flu?

In 1918 and 1919, an unusually virulent stain of avian flu killed at least 21 million people worldwide and perhaps as many as 50 million.

One theory behind the Spanish flu's sudden appearance involves pigs and chickens being raised in Kansas. The theory supposes that the pigs, already infected with one form of the influenza virus, became infected with avian flu from the chickens.

In such a situation, genes from one strain of the flu can become incorporated into another, giving rise to a new strain. This strain infected farmers, who infected soldiers at nearby Fort Riley, who were then deployed across the U.S. and Europe during the First World War.