Viruses: A Very Short Introduction (Very Short Introductions)

by Dorothy H. Crawford

When this finally comes, the union of receptor and antigen stimulates the lymphocyte to divide rapidly, forming a clone of cells with identical receptors. These are generally ready for action about a week after the initial infection.

One week after infection to form lymphocyte.

lymphocytes (or T cells) are the body’s single most important defence against viruses. There are two main types of T cells: helper T cells, characterized by the CD4 molecule on their surface, and killer (or cytotoxic) T cells, characterized by the CD8 molecule. Both CD4 and CD8 T cells kill virus-infected cells through the production of toxic chemicals that rupture the cell membrane, and CD4 T cells also produce cytokines that help CD8 T cells and B lymphocytes to grow, mature, and function properly.

Antibodies bind to viruses and virus-infected cells, helping to prevent spread of the invaders. In some instances, antibodies actually prevent viruses from infecting cells by blocking their receptor for entry and therefore are important in preventing later re-infection. The relative importance of T and B cells in the control of virus infections is well illustrated by rare mutations that wipe out one or other lymphocyte type.

generally protected from these infections during the first few months of life (as are healthy babies) by antibodies from their mother’s blood that cross the placenta in late pregnancy and are also present in breast milk.

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 not caused by the invading microbe itself but by the cytokines released by immune cells to fight it. On rare occasions, these immune-induced reactions may cause serious injury to internal organs, a result known as immunopathology.

HIV also infects and destroys CD4 T cells, the very cells that drive the immune response against it.

Most viruses induce solid immunity so that once recovered from an infection the host is resistant to further attack by the same virus. This naturally occurring immunity is mimicked by vaccines which may consist of killed or modified whole virus, or part of a virus.

Emerging infections engender fear sometimes verging on panic as an

Since the year 2000 we have seen the emergence of severe acute respiratory syndrome (SARS) (2003), a swine flu pandemic (2009), Middle East respiratory syndrome (MERS) (2012), an Ebola virus disease epidemic (2014), and a Zika virus epidemic (2016) as well as the threat of bird flu and the continuing human immunodeficiency virus (HIV) pandemic.

Emerging Viruhs Infections since 2000.

Examples of the former include swine flu and bird flu, as well as SARS and MERS coronaviruses. Many insect-transmitted viruses,

Novel viruses that emerge and spread successfully in a host population that has not previously met the virus may produce a small, local outbreak or a larger epidemic, both defined as ‘an infection occurring at a higher than expected frequency’, which may progress to a pandemic if it spreads on several continents at once.

differing patterns of emerging virus outbreaks depend on viral factors, including incubation period, disease manifestations, and method of spread, and important host factors like living conditions, propensity to travel, and the success of any preventive measures.

virus that jumps to a new host species for the first time has a series of hurdles to overcome before it can become established in the native population.

must infect cells of the new host, and this involves finding a host cell re...

Even if the virus can unlock and enter host cells, it still may not be able to reproduce inside them,

Nevertheless, on occasions viruses do enter and successfully replicate in cells of a new host species, but generally, after a window of opportunity lasting about a week during which they can colonize the host and reproduce, their offspring must move on to another susceptible host before the developing host immunity wipes them out.

another when the two come into contact. The latter are called zoonotic viruses, and the diseases they cause are zoonoses. Some zoonotic viruses can spread directly from their animal host to humans while others require an insect vector to ferry them between hosts.

Ebola disease virus (EDV) is a thread-like, RNA virus belonging to the Filovirus family. It was discovered after an explosive outbreak in Yambuku, a remote village in northern Zaire (now the Democratic Republic of the Congo, DRC), in 1976, and named after the local Ebola River.

haemorrhagic fever

until the culprit or culprits is/are found sudden, unexpected outbreaks of this lethal disease are likely to continue. Although there are no drugs to treat Ebola, an effective vaccine is now available and its use in an outbreak to immunize all contacts of the index case should curtail early virus spread.

Coronaviruses form a large family of viruses that mainly cause respiratory and gastrointestinal infections in humans, including the common cold,

Coronaviruses defined

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

July 2003 it was over, the final toll being around 8,000 cases and 800 deaths involving twenty-nine countries across five continents.

SARs deaths

SARS-CoV spreads through the air and after an incubation period of two to fourteen days victims develop fever, malaise, muscle aches, and a cough, sometimes progressing rapidly to viral pneumonia that requires intensive care, with mechanical ventilation in around 20 per cent of cases.

Sars symptons

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.

The search for an animal source for SARS-CoV focused on live animal markets in Gangdong. Here, there are a number of small mammals

natural reservoir of SARS coronavirus was later identified as the Chinese horseshoe bat, so it is presumed that the virus transferred from bats to other animal species in markets where they are packed together in overcrowded cages, and then jumped to the market traders, so sparking

Middle East Respiratory Syndrome coronavirus (MERS-CoV) is very similar to SARS-CoV, as is the disease it causes. The virus first emerged in Saudi Arabia in 2012,

By the end of 2016 there had been a total of 1,917 laboratory-confirmed MERS cases involving twenty-seven countries with an overall death rate of approximately 36 per cent.

MERS-CoV does not transmit easily between humans and although it occasionally appears to pass from patients to unprotected family members and carers, no sustained community transmission has been reported.

thought that camels are a major reservoir of the virus and the source of most human infections.

By the end of 2015 WHO reported 36.7 million people living with HIV globally, 70 per cent of whom are in sub-Saharan Africa.

78 million people have been infected with HIV, causing around 35 million deaths (Figure 10).

Untreated HIV infection leads to AIDS after an average of ten years, and this syndrome was first recognized in 1981 in San Francisco

people with multiple sexual partners, both heterosexual and homosexual; people with haemophilia and other disorders requiring regular infusions of blood or blood products; and injecting drug users.

is now clear that these HIV-like viruses have jumped from primates to humans in central Africa on several occasions in the past giving rise to human infections with HIV-1 types M, N, O, and P as well as HIV-2.

Influenza viruses (flu) are prime examples of viruses that mutate frequently, a process called antigenic drift.

regular winter outbreaks, with larger epidemics every eight to ten years. There are three flu strains, A, B, and C, but only flu A is a zoonotic virus.

Flu viruses are paramyxoviruses with an RNA genome with eight genes that are segmented, meaning that instead of being a continuous RNA chain, each gene forms a separate strand. The H (haemaglutinin) and N (neuraminidase) genes are the most important in stimulating protective host immunity.

last one hundred years, there have been five flu pandemics:

5 flu pandemics last 100 yrs.

H1N1 ‘Spanish’ flu of 1918, all eight genes came from birds; the H2N2 ‘Asian’ flu of 1957 acquired three new genes,

including H and N from birds; and the H3N2 ‘Hong Kong’ flu of 1968 acquired two new genes from wild ducks. The ‘Russian’ flu of 1977, which probably escaped from a lab in Russia, was a 1950s version of H1N1; whereas the H1N1 ‘swine’ flu which appeared in Mexico in 2009 has s...

By far the most virulent flu virus on record is the 1918 pandemic strain which targeted young adults and killed 40–50 million people worldwide,

An important mutation in a gene called NS1 prevents virus-infected cells from producing interferon, the key cytokine for preventing virus spread in the body and responsible for triggering the whole immune cascade.

panzootic (a pandemic in animals),

In the years since H5N1 bird flu emerged, several other virulent bird flu viruses have been recognized, all containing the H5 gene but with various different N genes, now all referred to as H5Nx, the highly pathogenic avian influenza (HPAI) viruses.

threat of a human pandemic remains as a genetic drift or shift could at any time generate a human transmissible virus.

a billion people worldwide boarding international flights every year, novel viruses have an efficient mechanism for rapid spread.

2015–16 Zika virus caused large epidemics of Zika fever in Brazil and other South American and Caribbean countries with millions of infections (Figure 12). Although the disease it caused was mild, reports of Zika infection in pregnant women resulting in severe birth defects in their babies and occasional cases of Guillain–Barré syndrome causing neurological damage in adults stimulated WHO to declare a Public Health Emergency of International

Concern in February 2016. Since