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When did you last have a cold that gave you a runny nose and made you sneeze? The common cold is one of hundreds of illnesses caused by viruses. The viruses are a group of micro-organisms, too small to be seen except through a special microscope called an electron microscope. In fact viruses are just about the smallest kinds of micro-organisms. Some are so small that 50,000 in a row would measure just one millimetre long. A pinhead-sized drop of water could contain 10 million viruses.
ARE VIRUSES ALIVE?
There are more than 3,500 main kinds of viruses. Some are simple rod shapes. Others are shaped like tiny golf balls, 20-sided boxes or tadpoles with a head and tail. All viruses cause diseases in other living things, because of the way they multiply. It is difficult to say if viruses themselves are really alive. Some kinds can be boiled or frozen, and turned into crystals, and show no signs of life for years. But this changes when the virus enters another living thing, called its host.
THE VIRUS INVADES
All living things are made of tiny building blocks called cells. Viruses have just one cell each. A virus is far smaller than the ordinary cells of plants and animals. It is smaller even than the one-celled micro-organisms known as bacteria. The viruses that specialize in attacking bacteria are known as bacteriophages or phages.
Most viruses are made of an outer covering formed from substances called proteins. This protein coat wraps around another substance inside, the genetic material. This is either DNA (deoxyribonucleic acid) or the similar RNA (ribonucleic acid). As in bigger living things, the virus’s genetic material contains instructions for surviving and making copies of itself. But viruses are not able to multiply by themselves. They need to get their genetic material inside a host cell in some other living thing before they can multiply. When a virus enters a cell of its host, it seems to come alive and takes off its protein coat.
HOW VIRUSES MULTIPLY
When the virus’s coat comes off, its genetic material can take over from the cell’s own genetic material. The cell follows new instructions—make more viruses. Within the host cell, copies of the virus’s genetic material are made, and new protein coats built for these to create new viruses. Soon the cell has built hundreds of viruses, which are copies of the original one. Usually the host cell is so damaged that it bursts open. The new viruses are set free to infect more cells, and so on. Sometimes the new viruses are released from the host by budding through the cell without killing it.
VIRUSES AND DISEASE
Usually, each type of virus infects only one kind of host. The first virus to be fully studied, in 1935, was TMV, tobacco mosaic virus. It causes patchy patterns on the leaves of tobacco plants. In people, different kinds of viruses cause many diseases, from cold sores (caused by one type of herpes simplex virus) and skin warts, to measles, mumps, rubella (German measles), chickenpox, shingles, flu (influenza), rabies, the liver disease hepatitis, the brain disease encephalitis, polio (poliomyelitis), yellow fever and some types of cancers. The HIV (human immunodeficiency virus) virus causes the very serious condition called AIDS (acquired immune deficiency syndrome).
NEW VIRUSES
A new kind of virus causes the illness SARS (severe acute respiratory syndrome) in people. A very similar virus has been found in mongoose-like animals called civets in East Asia. The virus may have changed, or mutated, into a new type or strain that then jumped to infect a different host—humans.
The abilities to mutate into new strains, and jump from one kind of living thing to another, are dangerous features of viruses. There are hundreds of strains of the common cold virus, and they are changing all the time. This is one reason scientists cannot develop a medical drug to treat the common cold.
USES FOR VIRUSES
The ability to jump from one kind of living thing to another may have produced the viruses that cause Lassa fever (a rare deadly disease) and AIDS. But this ability can also be helpful. Scientists use some viruses as carriers to take bits of genetic material from one kind of living thing to another. This is one method of genetic engineering or genetic modification to produce new forms of life.
THE BATTLE AGAINST VIRUSES
The changing nature of viruses means it is difficult to develop medical drugs to combat them. There are only a few kinds of these anti-viral drugs, compared to the many kinds of antibiotic drugs that fight bacteria.
One successful way of tackling viruses is vaccination. This usually involves giving injections (jabs or shots) against viral illnesses. For example, the MMR vaccine protects against measles, mumps and rubella. Vaccinations contain an altered version of a virus, which cannot multiply or cannot cause serious disease. When it enters the body, the body’s defences attack the virus. Then in future, if the real virus invades, the body’s defences will be prepared to act very quickly and kill it before it can multiply. This is known as being resistant, or immune, to the
When did you last have a cold that gave you a runny nose and made you sneeze? The common cold is one of hundreds of illnesses caused by viruses. The viruses are a group of micro-organisms, too small to be seen except through a special microscope called an electron microscope. In fact viruses are just about the smallest kinds of micro-organisms. Some are so small that 50,000 in a row would measure just one millimetre long. A pinhead-sized drop of water could contain 10 million viruses.
ARE VIRUSES ALIVE?
There are more than 3,500 main kinds of viruses. Some are simple rod shapes. Others are shaped like tiny golf balls, 20-sided boxes or tadpoles with a head and tail. All viruses cause diseases in other living things, because of the way they multiply. It is difficult to say if viruses themselves are really alive. Some kinds can be boiled or frozen, and turned into crystals, and show no signs of life for years. But this changes when the virus enters another living thing, called its host.
THE VIRUS INVADES
All living things are made of tiny building blocks called cells. Viruses have just one cell each. A virus is far smaller than the ordinary cells of plants and animals. It is smaller even than the one-celled micro-organisms known as bacteria. The viruses that specialize in attacking bacteria are known as bacteriophages or phages.
Most viruses are made of an outer covering formed from substances called proteins. This protein coat wraps around another substance inside, the genetic material. This is either DNA (deoxyribonucleic acid) or the similar RNA (ribonucleic acid). As in bigger living things, the virus’s genetic material contains instructions for surviving and making copies of itself. But viruses are not able to multiply by themselves. They need to get their genetic material inside a host cell in some other living thing before they can multiply. When a virus enters a cell of its host, it seems to come alive and takes off its protein coat.
HOW VIRUSES MULTIPLY
When the virus’s coat comes off, its genetic material can take over from the cell’s own genetic material. The cell follows new instructions—make more viruses. Within the host cell, copies of the virus’s genetic material are made, and new protein coats built for these to create new viruses. Soon the cell has built hundreds of viruses, which are copies of the original one. Usually the host cell is so damaged that it bursts open. The new viruses are set free to infect more cells, and so on. Sometimes the new viruses are released from the host by budding through the cell without killing it.
VIRUSES AND DISEASE
Usually, each type of virus infects only one kind of host. The first virus to be fully studied, in 1935, was TMV, tobacco mosaic virus. It causes patchy patterns on the leaves of tobacco plants. In people, different kinds of viruses cause many diseases, from cold sores (caused by one type of herpes simplex virus) and skin warts, to measles, mumps, rubella (German measles), chickenpox, shingles, flu (influenza), rabies, the liver disease hepatitis, the brain disease encephalitis, polio (poliomyelitis), yellow fever and some types of cancers. The HIV (human immunodeficiency virus) virus causes the very serious condition called AIDS (acquired immune deficiency syndrome).
NEW VIRUSES
A new kind of virus causes the illness SARS (severe acute respiratory syndrome) in people. A very similar virus has been found in mongoose-like animals called civets in East Asia. The virus may have changed, or mutated, into a new type or strain that then jumped to infect a different host—humans.
The abilities to mutate into new strains, and jump from one kind of living thing to another, are dangerous features of viruses. There are hundreds of strains of the common cold virus, and they are changing all the time. This is one reason scientists cannot develop a medical drug to treat the common cold.
USES FOR VIRUSES
The ability to jump from one kind of living thing to another may have produced the viruses that cause Lassa fever (a rare deadly disease) and AIDS. But this ability can also be helpful. Scientists use some viruses as carriers to take bits of genetic material from one kind of living thing to another. This is one method of genetic engineering or genetic modification to produce new forms of life.
THE BATTLE AGAINST VIRUSES
The changing nature of viruses means it is difficult to develop medical drugs to combat them. There are only a few kinds of these anti-viral drugs, compared to the many kinds of antibiotic drugs that fight bacteria.
One successful way of tackling viruses is vaccination. This usually involves giving injections (jabs or shots) against viral illnesses. For example, the MMR vaccine protects against measles, mumps and rubella. Vaccinations contain an altered version of a virus, which cannot multiply or cannot cause serious disease. When it enters the body, the body’s defences attack the virus. Then in future, if the real virus invades, the body’s defences will be prepared to act very quickly and kill it before it can multiply. This is known as being resistant, or immune, to the
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