Viruses: A Very Short Introduction (Very Short Introductions)

by Dorothy H. Crawford

Primitive microbes evolved on Earth approximately three billion years ago but were isolated by humans only in the late 19th century, around twenty years before Hilaire Belloc wrote ‘The Microbe’.

Pasteur was instrumental in dispelling the general belief in ‘spontaneous generation’, that is, the generation of life from inorganic material.

Koch discovered the first bacterium, Bacillus anthraci, in 1876. He soon developed methods for growing microbes in the laboratory and then, one after another, the causative microbes of feared diseases like anthrax, tuberculosis, cholera, diphtheria, tetanus, and syphilis were identified and characterized.

However, there remained a group of infectious diseases for which causative organisms could not be found, such as smallpox, measles, mumps, rubella, and flu. These microbes were obviously very small

In 1898, Martinus Beijerinck (1851–1931), from the Agricultural School in Wageningen, followed up on Mayer’s experiments by showing that the agent grew in dividing cells and regained its full strength each time it infected a plant. He concluded a living microbe was responsible, and was the first to coin the name virus, from the Latin meaning a poison, venom, or slimy fluid.

Coining the word "virus."

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’. The whole structure is called a virion and the outer coat is called the capsid.

Figure 1).

Illustration of virus types

Most viruses are too small to be seen under a light microscope. In general, they are around 100 to 500 times smaller than bacteria,

However, the recently discovered giant, the mimivirus (short for ‘microbe-mimicking virus’), is an exception, with a diameter of around 700 nm; larger than some bacteria.

Viruses usually have between 4 and 200 genes, but again mimivirus is most unusual in having an estimated 600 to 1,000 genes, even more than many bacteria.

Viruses, not being cells, have none of these and are therefore inert until

infect a living cell. Then they hijack a cell’s organelles and use what they need, often killing the cell in the process. Thus viruses are obliged to obtain essential components from other living things to complete their life cycle and are therefore called obligate parasites.

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.

well-known example is the AIDS virus, the human immunodeficiency virus (HIV) that carries the entry key for the CD4 lock, so only cells with CD4 molecules on their surface can be infected by HIV. This specific interaction defines the outcome of the infection, and in the case of HIV leads to destruction of CD4-positive ‘helper’ T cells that are critical to the immune response.

Once inside a cell, DNA viruses simply masquerade as pieces of cellular DNA, and their genes are transcribed and translated using as much of the cell’s machinery as they require.

The separate virus components are then assembled into thousands of new viruses which are often so tightly packed inside the cell that it eventually bursts open and releases them, inevitably killing the cell.

Retroviruses are a family of RNA viruses, including HIV, that have evolved a unique trick for establishing a lifelong infection of a cell while hiding from immune attack. Retrovirus particles contain an enzyme called reverse transcriptase which, once inside a cell, converts their RNA

Virus genomes mutate far more quickly than the human genome, partly because viruses reproduce in a day or two with many thousands of offspring.

Virus reprolduction razted.

Whatever the advantage, it will lead to that particular mutant virus outstripping its siblings and eventually taking over in the population. Examples

When a drug-resistant virus is transmitted to an uninfected person, the new infection is much more difficult to control. The same process has also foiled all attempts to make an effective HIV vaccine.

pox viruses of camels and gerbils, suggesting that all three arose from a common ancestor around 5,000 to 10,000 years ago.

Because virus particles are inert, without the ability to generate energy or manufacture proteins independently, they are not generally regarded as living organisms.

Given that modern-day viruses are obligate parasites, this theory proposes that large DNA viruses, for example pox viruses, may represent a previously free-living life form that has now lost its ability to reproduce independently.

DNA viruses evolved from their more ancient RNA counterparts.

Once it was appreciated that viruses carried either DNA or RNA, but never both,

the type of nucleic acid (DNA or RNA); • the shape of the virus capsid; • the capsid diameter and/or number of capsomeres; • the presence or absence of an envelope.

The hepatitis C virus discovery in 1989 was the first to use molecular probes.

either of these viruses. This disease was called non-A, non-B hepatitis, inevitably leading scientists to predict the existence of another

human gut, where there are so many bacteria that in the body overall they outnumber human cells by 12 to 1.

Of course, viruses, as obligate parasites, can only exist where life is found, so the question becomes, is there any life, microbial or otherwise, on other planets?

is a process that viruses have to leave entirely to chance as their particles are completely inert.

Viruses endure because they are so adaptable. Their fast reproduction rate and large number of offspring mean that they can evolve rapidly to meet changing circumstances.

Portals of virus entry into the human body.

Illustration: where viruses enter the body.

biting insects suck viruses up from one host and inject them into another while taking a blood meal.

dengue fever virus and yellow fever virus,

but a

microscopic abrasion is enough to allow entry of wart (papilloma) and cold sore (herpes simplex)

viruses, both very common infections caught directly from...

viruses that are too fragile to live for long outside their host’s body may be passed directly from one to another through close contact such as kissing. This is a very effective way of transmitting viruses in saliva, like Epstein–Barr virus which causes glandular fever, also known as ‘the kissing disease’. Some viruses like HIV and hepatitis B (HBV) make use of the sexual route

Epstein Barr

viruses even exploit modern interventions like blood transfusion and organ transplantation, and may also contaminate surgical instruments and dentists’ drills, so allowing them to jump from one host to another.

protective mechanism, first identified in plants but also used by insects and other animal species, is gene silencing by RNA interference (RNAi).

short RNA molecules that are found inside cells of most species, including humans, where they regulate the manufacture of proteins by binding

RNA messages and preventing their translation in...

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.

CCR5 that codes for a protein that is essential for HIV infection. About 10 per cent of the Caucasian population has a deletion

Scientists think that the CCR5 deletion must have conferred a selective advantage in the past by protecting against a lethal microbe, with plague and smallpox being strong contenders as they have both been major killers for over 2,000 years.

Viruses often gain access to the body by infecting cells of the respiratory,

intestinal, or genitourinary tracts, the deeper layers of the skin, and the surface of the eye, and may then disseminate from these areas to infect internal organs. At the site of entry infected cells send out

interferon, which renders surrounding cells resistant to infection at the same time as alerting the immune system to the invasion and kickstarting the immune response ...