Viruses: A Very Short Introduction (Very Short Introductions)

by Dorothy H. Crawford

Once inside, they hijack the cell’s organelles and use what they need to complete the virus life cycle, often killing the cell in the process. As such they are called obligate parasites.

Plant viruses either enter cells through a break in the cell wall or are injected by sap-sucking insect vectors like aphids. They then spread very efficiently from cell to cell via plasmodesmata, pores that transport molecules between cells.

animal viruses infect cells by binding to specific cell surface receptor molecules. The cell receptor is like a lock, and only viruses that carry the right receptor-binding key can open it and enter that particular cell.

every time a virus infects a cell, its DNA or RNA may be copied thousands of times, and as each new strand is incorporated into a new virus particle, every round of infection throws up several mutant viruses. This high mutation rate in viruses is their lifeline; in some, it is essential for their survival.

Because viruses have a high mutation rate, significant evolutionary change, estimated at around 1 per cent per year for HIV, can be measured over a short timescale.

We now know that viruses have invaded every niche where life exists, including the most inhospitable places like hydrothermal vents, under the polar ice caps, and in salt marshes and acid lakes.

Natural, untreated water is teeming with viruses and, in fact, viruses are the most abundant life forms in the oceans. With up to 10 billion of them per litre of sea water, the whole ocean contains around viruses.

despite their tiny size, viruses are of prime importance in the stability of ecosystems worldwide.

The battle between humans and microbes has been ongoing ever since humans evolved, with microbes developing new means of attack and our immune system retaliating with improved defences in an escalating arms race. As a virus’s generation time is so much shorter than ours, the evolution of genetic resistance to a new human virus is painfully slow, and constantly leaves viruses with the advantage.

At the height of its activity, the immune response may be so pronounced that it actually does harm to the body. In fact, the typical, non-specific symptoms we experience with an acute dose of flu, such as fever, headache, enlarged tender glands, and general fatigue, are often caused not by the invading microbe itself but by the cytokines released by immune cells to fight it.

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

Inoculation was introduced to Britain in the 1720s by Lady Mary Wortley Montagu (1689–1762), who saw it performed while living in Constantinople (now Istanbul, Turkey)

The HIV pandemic has been ongoing among humans since the early 1900s, and despite drugs that control the infection, it is still raging in many parts of the world, most particularly in sub-Saharan Africa.

Since the first identification of HIV-induced AIDS it has accounted for approximately 36.3 million deaths.

Untreated HIV infection leads to AIDS after an average of 10 years, and this syndrome was first recognized in 1981 in San Francisco when several gay men died of unusual infections superimposed on severe HIV-induced immunosuppression.

Using stored blood samples to track back to the origin of HIV in humans, sub-Saharan Africa, particularly Kinshasa in DRC, was pinpointed as the epicentre of the pandemic. Scientists calculated that HIV has infected people in this region for approximately 100 years and that around 1966 a single virus strain inside an infected person was carried from DRC to Haiti where it spawned an epidemic.

So by the time HIV was discovered in 1983, the pandemic was already growing exponentially and has proved very difficult to control.

The first human coronavirus (HCoV) was isolated in 1961 during the search for the common cold virus.

Once an acute emerging virus such as a new strain of flu is successfully established in a population, it generally settles into a mode of cyclical epidemics during which many susceptible people are infected and either die or become immune to further attack. When herd immunity is reached, the virus moves on, only returning when a new susceptible population has emerged, which generally consists of those born since the last epidemic.

Studies on measles outbreaks in island populations of varying sizes, such as Iceland, Greenland, Fiji, and Hawaii, show that a population of around 500,000 is required for the virus to circulate continuously in a community, a figure that is probably similar for other airborne viruses. The first towns of this size evolved around 5000 bc in the Fertile Crescent, and so sometime after this date, viruses like measles were able to break the link with their animal hosts to become entirely human pathogens.

Viruses spread between hosts in many different ways, but those that cause acute epidemics generally utilize fast and efficient methods, such as the airborne or faecal–oral routes.

Although AIDS was first described in gay men in the USA, and shortly afterwards injecting drug users and haemophiliacs were found to be at risk, worldwide the virus is mainly transmitted by heterosexual intercourse.

over 37 million people were recorded living with HIV in sub-Saharan Africa in 2021.

the worst-hit countries in sub-Saharan Africa, where life expectancy tumbled to below 40 years in the 1990s, caused by the wholesale death of previously healthy and productive adults, creating an economic downturn and severe poverty. Life expectancy has only risen to an average of 54 years some 30 years later.

So far, all the human tumour viruses discovered are persistent viruses that successfully evade their hosts’ immune attack and remain on board long-term.

The link between HPV and cervical cancer was discovered by Harald zur Hausen, a German virologist from Nuremberg who won a Nobel Prize for this work in 2008.

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.

Beginning in 2008, a vaccine against HPV 16 and 18 was offered to boys and girls aged 12‒13 years in the USA and Europe, followed by a catch-up programme in girls aged 14‒18 years. By 2021, a UK study found a reduction in cervical cancer ranging from 34 per cent in those vaccinated at 16‒

18 years of age to 87 per cent in those vaccinated at 12‒13 years of age. In addition, the corresponding risk reductions for CINIII were 39 per cent in the 16‒18 age group and 97 per cent in the 12‒13 age group.

viruses themselves are ancient parasites whose history and evolution is closely entwined with our own.

The world’s population approximately doubled every 500 years between the beginning of the Common Era and 1900, when it reached 1.6 billion. But in the 20th century, life expectancy rose steeply and the population quadrupled, hitting 6 billion by 2000. If this growth rate continues unabated, we are set to reach 9–10 billion by 2100.

Animal viruses also thrive on overcrowding and travel. For them, intensively farmed animals equate to teeming megacities, presenting the opportunity for easy spread.