A virus is a biological
agent that reproduces inside the cells of living hosts.. When infected by a
virus, a host cell is forced to produce many thousands of identical copies of
the original virus, at an extraordinary rate. Unlike most living things, viruses
do not have cells that divide; new viruses are assembled in the infected host
cell. But unlike still simpler infectious agents, viruses contain genes, which
give them the ability to mutate and evolve. Over 5,000 species of viruses have
been discovered.
The origins of viruses are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. A virus consists of two or three parts: genes, made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects the genes; and in some viruses, an envelope of fat that surrounds and protects them when they are not contained within a host cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses range in size from 20 to 300 nanometres; it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimetre (0.39 in).
Viruses spread in many ways. Just as many viruses are very specific as to which host species or tissue they attack, each species of virus relies on a particular method for propagation. Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors. Some viruses of animals, including humans, are spread by exposure to infected bodily fluids. Viruses such as influenza are spread through the air by droplets of moisture when people cough or sneeze. Viruses such as norovirus are transmitted by the faecal-oral route, which involves the contamination of hands, food and water. Rotavirus is often spread by direct contact with infected children. The human immunodeficiency virus, HIV, is transmitted by bodily fluids transferred during sex. Others, such as the Dengue virus, are spread by blood-sucking insects.
Viral infections can cause disease in humans, animals and even plants. However, they are usually eliminated by the immune system, conferring lifetime immunity to the host for that virus. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent some viral infections.
Common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Serious diseases such as Ebola and AIDS are also caused by viruses. Many viruses cause little or no disease and are said to be "benign". The more harmful viruses are described as virulent. Viruses cause different diseases depending on the types of cell that they infect. Some viruses can cause lifelong or chronic infections where the viruses continue to reproduce in the body despite the host's defence mechanisms. This is common in hepatitis B virus and hepatitis C virus infections. People chronically infected with a virus are known as carriers. They serve as important reservoirs of the virus. If there is a high proportion of carriers in a given population, a disease is said to be endemic.
There are many ways in which viruses spread from host to host but each species of virus uses only one or two. Many viruses that infect plants are carried by organisms; such organisms are called vectors. Some viruses that infect animals, including humans, are also spread by vectors, usually blood-sucking insects. However, direct transmission is more common. Some virus infections, (norovirus and rotavirus), are spread by contaminated food and water, hands and communal objects and by intimate contact with another infected person, while others are airborne (influenza virus). Viruses such as HIV, hepatitis B and hepatitis C are often transmitted by unprotected sex or contaminated hypodermic needles. It is important to know how each different kind of virus is spread to prevent infections and epidemics.
The origins of viruses are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. A virus consists of two or three parts: genes, made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects the genes; and in some viruses, an envelope of fat that surrounds and protects them when they are not contained within a host cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses range in size from 20 to 300 nanometres; it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimetre (0.39 in).
Viruses spread in many ways. Just as many viruses are very specific as to which host species or tissue they attack, each species of virus relies on a particular method for propagation. Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors. Some viruses of animals, including humans, are spread by exposure to infected bodily fluids. Viruses such as influenza are spread through the air by droplets of moisture when people cough or sneeze. Viruses such as norovirus are transmitted by the faecal-oral route, which involves the contamination of hands, food and water. Rotavirus is often spread by direct contact with infected children. The human immunodeficiency virus, HIV, is transmitted by bodily fluids transferred during sex. Others, such as the Dengue virus, are spread by blood-sucking insects.
Viral infections can cause disease in humans, animals and even plants. However, they are usually eliminated by the immune system, conferring lifetime immunity to the host for that virus. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent some viral infections.
Viruses and Diseases
Common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Serious diseases such as Ebola and AIDS are also caused by viruses. Many viruses cause little or no disease and are said to be "benign". The more harmful viruses are described as virulent. Viruses cause different diseases depending on the types of cell that they infect. Some viruses can cause lifelong or chronic infections where the viruses continue to reproduce in the body despite the host's defence mechanisms. This is common in hepatitis B virus and hepatitis C virus infections. People chronically infected with a virus are known as carriers. They serve as important reservoirs of the virus. If there is a high proportion of carriers in a given population, a disease is said to be endemic.
There are many ways in which viruses spread from host to host but each species of virus uses only one or two. Many viruses that infect plants are carried by organisms; such organisms are called vectors. Some viruses that infect animals, including humans, are also spread by vectors, usually blood-sucking insects. However, direct transmission is more common. Some virus infections, (norovirus and rotavirus), are spread by contaminated food and water, hands and communal objects and by intimate contact with another infected person, while others are airborne (influenza virus). Viruses such as HIV, hepatitis B and hepatitis C are often transmitted by unprotected sex or contaminated hypodermic needles. It is important to know how each different kind of virus is spread to prevent infections and epidemics.
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