Section: Science

191) Excerpt from http://www.bbc.co.uk/science/0/21143412

Why can’t we beat viruses?

Every winter, cold and flu sweep across the country, bringing aches, runny noses and fever to millions. The flu virus can kill vulnerable people and a full-blown pandemic can cause a national health crisis.

Scientists have been studying viruses for years. Some, like measles and smallpox, can be defeated with vaccines and drugs.

But some viruses return every year to wreak havoc. So why are they so hard to beat?

How a virus works

Viruses are about a hundred times smaller than human cells. They come in many different shapes and are present wherever there are cells to infect. In fact, viruses are the most common biological unit on Earth, outnumbering all other types put together.

When a virus enters our body it tries to attack a cell.

If our immune system recognises the virus as an intruder, it will be destroyed before the virus can gain entry to a cell. If not, the process of infection begins.

Once inside the cell, the virus can hijack the cell's own replication machinery, which starts to make many copies of the virus. These viruses burst out of the cell, destroying it, and will attempt to infect many more cells unless tackled by the immune system. The infection can also start to spread to other people.

This can happen fast, with devastating consequences. The 1918 Spanish flu was one of the most aggressive viral pandemics in recent history. It is thought to have caused up to 50 million deaths worldwide.

Stopping the virus in its tracks

The human immune system is remarkably effective at dealing with viral invaders.

Some of the symptoms caused by viruses - such as fever, vomiting and tiredness - are a result of the body's defence mechanisms. In many cases of flu and cold the immune system destroys the infection.

The immune response leads to something called acquired immunity - the body 'remembers' viruses so that it can quickly destroy them, should they return, and also make us resistant to them in future. A single case of measles as a child, for example, gives us lifelong resistance.

Scientists have used this concept to protect people against viruses, with some notable success. By 'tricking' our immune systems into recognising viruses, without actually getting infected, we can develop effective resistance to real viruses.

In the 17th century, a method of smallpox immunisation saw people snort or rub dried smallpox scabs into their skin to create an immune response, which then protected them from contracting the virus.

Then in 1796, British physician Edward Jenner developed the first vaccine (for smallpox) which led to a complete eradication of the disease by 1980.

This was a milestone in the history of medical advances. For the first time ever, the World Health Organisation announced the total global eradication of a once deadly disease.

So why can't vaccination work against all types of viruses?

Our immune system works by recognising the proteins on the surface of the virus. But in certain types of virus, these proteins keep changing as the virus mutates, so a vaccine developed one year might not work the next.

Viruses like HIV have proved impossible to develop any kind of vaccine for at all.

Drugs are another line of defence. Unfortunately, they are less effective against viruses than they are against bacteria. Antibiotic drugs kill bacteria by disrupting their cell walls. But virus' external covering, known as the viral envelope, is almost identical to the host cell's membranes, making them difficult to target.

Some anti-viral drugs have been developed. They work by disrupting a part of the virus' lifecycle. Some interfere with the virus' attachment to the cell; others stop the virus' genetic material being incorporated inside the host cell.

Used in combination, some antiviral drugs have proved effective against specific viruses. Anti-viral drugs can dramatically prolong the lives of people affected by HIV.

What we can do to reduce the risk?

Because of the limitations of vaccines and drugs, doctors and health protection agencies often recommend 'good practice' aimed at reducing the chance of infection. Staying at home when infected will avoid spreading the infection to others, and minimising contact with virus particles by thoroughly washing hands and cleaning surfaces will limit the spread of the virus to others.

Viruses are incredibly simple organisms but are extremely effective at infecting us. We may never be able to totally beat viruses, but by harnessing the power of vaccination, the latest anti-virals and adopting the right behaviour, we stand a good chance of keeping one step ahead of viruses' most harmful effects.

What is the most common biological unit on earth, according to the passage?

Explanation

Explained in paragraph immediately following "How a virus works" section.
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