Viruses: A Very Short Introduction (Very Short Introductions)

by Dorothy H. Crawford

Viruses are not cells but particles. They consist of a protein coat which surrounds and protects their genetic material, or, as the famous immunologist Sir Peter Medawar (1915–87) termed it, ‘a piece of bad news wrapped up in protein’.

Most viruses are too small to be seen under a light microscope. In general, they are around 100 to 500 times smaller than bacteria, varying in size from 20 to 300 nanometres in diameter (nm; 1 nm is a thousand millionth of a metre)

Microbes are by far the most abundant life form on Earth. Globally, there are about 5 × 1030 bacteria, and viruses are at least ten times more common—thus making viruses the most numerous microbes on Earth. Indeed, there are more viruses in the world than all other forms of life added together. Viruses are also staggeringly diverse, with an estimated 100 million different types.

Coronaviruses form a large family of viruses that mainly cause respiratory and gastrointestinal infections in humans, including the common cold, and also infect a wide variety of other mammals. Severe Acute Respiratory Syndrome coronavirus (SARS-CoV) first emerged in November 2002 in Foshan, Guangdong Province, China, where it caused an outbreak of atypical pneumonia. Initially, the virus spread locally, particularly among patients’ family members and hospital staff, but everything changed in February 2003 when a doctor who had treated SARS cases in Guangdong Province travelled to Hong Kong. He stayed one night in a hotel before being admitted to hospital where he died of SARS a few days later. The virus spread to hospital staff and visitors, sparking an epidemic in Hong Kong (Figure 9). The doctor also passed SARS-CoV to at least seventeen hotel guests who then carried it to five more countries, spawning epidemics in Canada, Vietnam, and Singapore.

Left to its own devices, SARS-CoV would undoubtedly have continued its trail of destruction but many of its characteristics contributed to its speedy demise. Importantly, the virus mostly causes overt disease so cases and their contacts could be recognized and isolated, and since victims are only infectious once the symptoms have developed, this prevented further spread.

Before vaccination programmes became widespread, young children suffered from a series of well-recognized infectious diseases called the ‘childhood infections’. These included measles, mumps, rubella, and chickenpox, all caused by viruses, of which only chickenpox remains widespread in the West today.

Smallpox virus is in a class of its own as the world’s worst killer virus. It first infected humans at least 5,000 years ago and killed around 300 million in the 20th century alone. The virus killed up to 30 per cent of those it infected, scarring and blinding many of the survivors. But after centuries of devastation, smallpox virus was finally eliminated from the wild in 1980. The fight to prevent and eliminate smallpox is recounted in Chapter 9

Of the three viruses, measles is the most infectious and produces the severest disease. It killed millions of children each year before vaccination was introduced in the mid-20th century. Even today, this virus kills over 70,000 children annually in countries with low vaccine coverage.

Worldwide, 10–20 per cent of human cancers are linked to viruses, including some common tumours like cervical cancer in women and liver cancer, which is more common in men.