Viruses: A Very Short Introduction (Very Short Introductions)

by Dorothy H. Crawford

in the 1990s showed that those persistently infected with HBV were over 200 times more likely than non-carriers to develop liver cancer, and that over half the deaths in this group were due to liver cancer or cirrhosis.

Since the tumour develops many years after the initial infection, several rare events must be required for tumour outgrowth.

Finally, certain toxins that may contaminate poorly preserved food can cause liver cancers in animals.

human papilloma viruses (HPVs), a very large family of viruses with over 100 different

and verrucae, are harmless, a few types can cause cancer, most commonly cancer of the uterine cervix in women.

The link between HPV and cervical cancer was suggested in the 1970s by Harald zur Hausen, a German virologist from Nuremberg who then went on to prove the association and win a Nobel

now know that HPV DNA, particularly from types 16 and 18, is present in the cells of almost all cervical cancers, as well as the less common cancers of the vagina, vulva, penis, anus, skin, mouth, throat, and larynx.

Indeed, tests on 18- to 25-year-old healthy American women show that up to 46 per cent carry HPV, of which types 16 and 18 account for around one-third. Furthermore, regular screening for

vaccine against HPV 16 and 18 is now being offered to pre-teenage boys and girls in the USA and continental Europe (see Chapter 9) in the belief that preventing infection with the two most oncogenic HPVs will have a dramatic effect on the incidence of cancer of the cervix in years to come.

present, 1.8 million virus-associated cancers are diagnosed worldwide annually. This accounts for 18 per cent of all cancers, but since these human tumour viruses were only identified fairly recently, it is probable that there are several more out there waiting to be discovered.

The first recorded way of preventing smallpox was inoculation, used in China and India for hundreds of years before it reached Western Europe in the 1700s.

incubation period of around two weeks provided

And as smallpox virus is a stable DNA virus with only one major type, there was little likelihood of it mutating into a vaccine-resistant strain.

aim was to increase vaccination coverage to over 80 per cent, the critical level for preventing virus spread.

Worldwide, eradication of smallpox was declared in 1980.

This is known as ‘post-exposure’ vaccination and it works because the virus must follow nerve pathways from the site of infection to the brain before causing symptoms.

So as long as the vaccine is given soon after the bite, it should prevent the virus reaching the brain.

just before the inactivated vaccine produced by American virologist Jonas Salk (1914–95) came into use. It had an immediate effect, reducing the number of polio cases in the USA from 20,000 to around 2,000 per year. However, it had to be given by injection, and at first it was of fairly low potency.

Albert Sabin (1906–93), manufactured a live attenuated polio vaccine that became available in the early 1960s. He grew the virus in the laboratory until a weakened strain emerged that induced immunity without causing disease.

WHO Polio World Eradication Campaign begun in 1988 aimed to achieve over 80 per cent coverage with oral vaccine.

There are several reasons for these failures. First, HIV mutates rapidly, and after around a hundred years of human infection there are many different types and strains that may not all be prevented by a single vaccine preparation. Secondly, HIV persists in everyone it infects, indicating that the natural immune response against it cannot clear the virus.

Thirdly, HIV is usually transmitted via the lining of the genital tract, so antibodies and immune T cells in the blood must reach this site to prevent HIV infecting CD4 cells and establishing latent infection.

HIV may be transmitted either a...

Once it was realized that viruses are parasites that grow inside cells, cell culture techniques were developed for their propagation and isolation. These included growth in hens’ eggs and in cultured cells, both of which showed a virus-induced cytopathic effect (CPE) on virus-infected cells which is characteristic of specific viruses.

Diagnostic laboratories are still unable to find a culprit virus in many so-called ‘viral’ meningitis, encephalitis, and respiratory infections. This strongly suggests that there are many pathogenic viruses waiting to be discovered, and

herpesvirus cytomegalovirus (CMV), found as a persistent infection in approximately 50 per cent of the developed world’s population, which has been linked to coronary heart disease. The virus can be found in atheromatous plaques in diseased arteries where the chronic inflammation it causes may contribute to the subsequent blockage of blood flow that precipitates a heart attack. These intriguing associations certainly warrant further investigation. As we have seen with cancer, although viruses may represent only one link in the chain of events that leads to a disease, their removal could prevent the disease occurring.

virus escape is not unprecedented—the flu virus that escaped from a Russian laboratory and caused a pandemic in 1977 is a case in point (see Chapter 4), and, astonishingly, the last cases of smallpox occurred when the virus escaped from a microbiology lab at Birmingham University, UK, in 1978.

We should heed the warning of the late virologist George Klein: The stupidest virus is cleverer than the cleverest virologist.