Introduction to viruses
When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus.
But unlike simpler infectious agents like prions, they contain genes, which allow them to mutate and evolve.
Some viruses may also have an envelope of fat-like substance that covers the protein coat, and makes them vulnerable to soap.
Some examples of such "zoonotic" diseases include coronavirus in bats, and influenza in pigs and birds, before those viruses were transferred to humans.
His experiments showed that extracts from the crushed leaves of infected tobacco plants remain infectious after filtration.
[6] In 1935, American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus (TMV) and found it to be mainly made from protein.
The breakthrough came in 1931, when American pathologists Ernest William Goodpasture and Alice Miles Woodruff grew influenza, and several other viruses, in fertilised chickens' eggs.
This problem was solved in 1949, when John Franklin Enders, Thomas Huckle Weller, and Frederick Chapman Robbins grew polio virus in cultures of living animal cells.
These techniques rely on the availability of ancient viral DNA or RNA, but most viruses that have been preserved and stored in laboratories are less than 90 years old.
Some viruses are surrounded by a bubble of lipid (fat) called an envelope, which makes them vulnerable to soap and alcohol.
Cells produce new protein molecules from amino acid building blocks based on information coded in DNA.
Some viruses that infect animals, including humans, are also spread by vectors, usually blood-sucking insects, but direct transmission is more common.
[64] Throughout history, human migration has aided the spread of pandemic infections; first by sea and in modern times also by air.
[61][70][71] Restrictions unprecedented in peacetime were placed on international travel,[72] and curfews imposed in several major cities worldwide.
When control of plant virus infections is considered economical (perennial fruits, for example) efforts are concentrated on killing the vectors and removing alternate hosts such as weeds.
[76] They are important in marine ecology: as the infected bacteria burst, carbon compounds are released back into the environment, which stimulates fresh organic growth.
The skin of animals, particularly its surface, which is made from dead cells, prevents many types of viruses from infecting the host.
When a virus overcomes these barriers and enters the host, other innate defences prevent the spread of infection in the body.
[78] Specific immunity to viruses develops over time and white blood cells called lymphocytes play a central role.
Lymphocytes retain a "memory" of virus infections and produce many special molecules called antibodies.
After the infection subsides, some antibodies remain and continue to be produced, usually giving the host lifelong immunity to the virus.
[81] When they are infected, plants often produce natural disinfectants that destroy viruses, such as salicylic acid, nitric oxide and reactive oxygen molecules.
These enzymes, called restriction endonucleases, cut up the viral DNA that bacteriophages inject into bacterial cells.
Their use has resulted in the eradication of smallpox and a dramatic decline in illness and death caused by infections such as polio, measles, mumps and rubella.
[88] Biotechnology and genetic engineering techniques are used to produce "designer" vaccines that only have the capsid proteins of the virus.
[93] Treatments for chronic carriers of the hepatitis B virus have been developed by a similar strategy, using lamivudine and other anti-viral drugs.
[94] HIV infections are usually treated with a combination of antiviral drugs, each targeting a different stage in the virus's life cycle.
They infect and destroy the bacteria in aquatic microbial communities and this is the most important mechanism of recycling carbon in the marine environment.
[103] Many other viruses, including caliciviruses, herpesviruses, adenoviruses and parvoviruses, circulate in marine mammal populations.
[102] Viruses can also serve as an alternative food source for microorganisms which engage in virovory, supplying nucleic acids, nitrogen, and phosphorus through their consumption.
