There are two different definitions of the word "virus". One is centuries old, the other is a modern hijacking of the concept.

But there are a surprising number of parallels between human and computer viruses.

Dr Rod Daniels, who heads a virology research laboratory at the National Institute for Medical Research, has worked with Jack Clark, antivirus technology consultant for McAfee, to identify them.

The most striking are:

Both types of virus are made
up from strings of very basic components.

Medical viruses are made up from nucleotides (a type of chemical compound) while computer viruses are made up from thousands of bits of data.

A typical influenza virus is made up from 13,700 nucleotides making it a 13.7kb (kilobase) virus.

Both medical and computer
viruses can be measured in terms of their "pathogenicity".

This simply means they are measured according to the damage they can do.

Certain medical viruses are lethal to elderly people while youngsters shrug them off.

Similarly some older operating systems can be wrecked by a computer virus while newer systems have inbuilt resistance.

Both types of viruses are
"parasites" and need a host in which to reproduce.

Medical viruses cannot exist in isolation and need a host (a living cell) in which to reproduce. In medical terms, this is known as being an obligate parasite.

Virtually all computer viruses are also parasitic and hide themselves within either a commonly-used programme or file which then enables them to gain access to a new computer system.

Both types of virus are
spread by personal contact.

The most efficient way for medical viruses to spread is through the air breathed by donors and recipients. Most computer viruses are spread by emails.

Shared screensavers and egreetings cards are often at the root of computer virus replication.

The most dangerous
medical and computer viruses are often the smallest.

A large number of highly contagious and potentially lethal medical viruses have relatively small genome sizes.

These range from 7.4Kb to 19Kb.

At the other end of the spectrum, the smallpox virus, variola (which was declared eradicated in 1980), had a genome of about 190Kb.

This situation is reflected in the world of computer viruses.

Before email usage became common, viruses had to fit on to the boot sector of a computer which holds information on locations, sizes and names of the files on a disc.

This area is a fraction of the disc size. Email allows viruses to attach large chunks of computer code to files before they are sent out.

Many viruses (both medical
and computer) originate in the East.

Many medical viruses are passed to humans through pigs, chickens and aquatic birds. Because Asia is so densely populated and people live close to their livestock, virus spread is exacerbated.

Asia was the source of pandemic strains of influenza virus in 1957 and 1968 and also where the potentially-lethal H5N1 virus entered the human population in 1997.

Computer viruses also move from East to West. They follow the sun. This is because they are generally spread as the user turns on his or her computer in the morning.

So as each part of the world wakes up, it takes in and activates new viruses, causing spread before a "fix" can be implemented. A virus has to originate in Asia and build momentum before it hits the United States where most of the world's mission critical servers are based.

The Love Letter and Nimda viruses were both examples.

The Melissa virus, by contrast, was created in the United States.

Both medical and computer
viruses are tracked in the same way.

Virus fighters such as Network Associates and the World Health Organisation have almost identical methods of tracking virus spread.

Both work globally and both have centres at strategic points around the world where information is gathered and disseminated.