Viral diseases are caused by viruses, which are particles with a core of genetic material, either DNA or RNA, and a protein coat. They can only reproduce inside living cells. Viruses come in many shapes and sizes, but all of them have the same basic structure.
Viruses can enter the body through mucous membranes in the nose, eyes, mouth, anus or genitals. They also can be transmitted by a mosquito or tick bite. Unlike bacteria, most viruses are specific about the cells they infect.
Viruses are tiny
Viruses are the smallest of all microorganisms, and they can only reproduce inside living cells. They are also the smallest infectious agents. Viruses are the reason why we get the common cold, the flu, and many other diseases. They are so small that you can’t even see them with a microscope. The study of viruses is known as virology.
Unlike bacteria, viruses don’t have the chemical machinery necessary to carry out life’s processes. Instead, they must attach to host cells (a cell can be an animal, plant, or bacterium) and hijack their normal functions to make more viruses and to grow. Eventually, the host cell will explode or run out of energy and die.
The simplest viruses have a core of genetic material, either RNA or DNA, surrounded by a protein coat called a capsid. The capsid is sometimes enclosed in an envelope, which gives it a spikey appearance and allows the virus to latch onto cells. The envelope is a lipid layer, which allows the virus to penetrate a cell’s membrane.
Most viruses use a strategy that involves receptor binding and direct fusion. Receptor binding happens when a cell’s proteins detect the presence of viruses and open the door for them to enter. Infection can also happen when viruses attach themselves to cells and inject their genetic material into them. This is called lysogenic replication.
Some viruses have a dormant phase when they are in a host cell. During this time, the cells they infect are not cooking, but the viruses are storing their recipe inside them. Then, when the right triggers occur, they will burst, releasing viral particles into the body and infecting other cells.
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Viruses are so simple that they don’t have their own cellular machinery to do anything, so they must take advantage of a cell’s replication mechanisms to multiply and cause disease. For this reason, many scientists believe that they aren’t considered to be alive. However, some argue that because they are made from the same building blocks of life as cells, they are on the edge of being living things.
They need a host to reproduce
Viruses cannot live outside of cells because they lack the metabolic processes that allow other living things to survive independently. They also can’t capture or store energy to reproduce, so they must invade cells in order to survive and multiply. This complete dependency on a host organism has led many scientists to consider viruses as non-living. Despite this, they are still capable of reproducing in their host, and they can even kill the cell that they’re infected with.
A virus consists of nucleic acid, either DNA or RNA, and a protein coat that encloses it. A virus can infect living cells and make them produce more viruses. This process is called a lytic cycle.
Unlike bacteria, which can infect a variety of different species, most viruses are specific about the kinds of cells they infect. In some cases, viruses can turn normal cells into malignant (cancerous) cells or cancerous ones. Viruses can be very dangerous to humans because they infect the cells in many parts of the body, including the liver, blood, and brain. Viruses can enter the body through the nose, mouth, eyes, anus, or genitals; through contact with infected animals or insect bites; by eating or drinking contaminated water; and via airborne particles. Viruses can also be passed from a mother to her fetus, or may be transmitted during labor and delivery.
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Infection by a virus can lead to a range of symptoms, from asymptomatic to life-threatening. Symptoms can include fever, vomiting, nausea, and diarrhea. Most viral infections can be diagnosed by observing the patient’s behavior, blood tests and cultures, and examination of the infected tissues. Treatment usually involves taking antiviral drugs to interfere with the reproduction of the virus, or to strengthen the immune response.
Viral diseases can spread from person to person through coughing or sneezing, by touching surfaces that have been touched by someone who is infected; by vaginal, oral or anal sex; or by the inhalation of fumes from smoking tobacco or chemicals. Viruses can also be passed to babies through the vagina, mouth or anus during birth and by the mother’s milk.
They can live outside of a host
Viral pathogens lack the ability to reproduce on their own, so they must infiltrate and replicate copiously inside host cells. To do so, viruses must be able to access the protein synthesis machinery of host cells. This process is regulated by a complex network of molecular mechanisms, including cellular polarity and membrane dynamics. This is an important factor in determining virus susceptibility and the ability to cause disease.
Although biologists do not consider viruses to be living organisms, they are well-adapted to their hosts and have the capacity to spread rapidly. They can do so through many portals of entry, modes of spread, and targets for replication. They can also alter the genetic material of host cells and trigger mutations. Infections by virus strains that cause disease can be mild or severe, depending on the target organ and virulentness of the virus.
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Despite their tiny size, viruses are able to survive outside of a cell for an indeterminate period. This time varies by pathogen, but the length of incubation is influenced by physical barriers, temperature and other environmental factors. For example, noroviruses, which can cause diarrhea and vomiting, can remain infectious for weeks on surfaces. The length of time a virus remains infectious also depends on how much of the virions are bound to surface proteins, such as the capsid and envelope proteins, as well as any structures that stick out from the capsid (like spikes) that help them enter cells.
A virus’s structure consists of a core of nucleic acid, or RNA, and an outer shell, or capsid. The capsid is made of structural proteins, called capsomeres, which are stacked in a helix or other shape. The capsid may also contain an envelope of lipids, which protects the viral nucleic acid from water and other substances. The structures of the capsid and envelope vary between different viruses, but many are shaped like spheres or rods.
Infection is caused when a molecule on the virus’s surface matches a receptor on the surface of a host cell, much like a key fitting into a lock. Once the virus infiltrates a cell, it hijacks its protein synthesis machinery to produce a large number of copies of itself. During this process, the host cell may rupture or die.
They cause disease
A virus is a tiny infectious organism that requires a host cell in order to reproduce. It consists of nucleic acid (DNA or RNA) surrounded by a protein coat and lipid envelope. It can be transmitted from person to person by air, food, saliva, bites, and contaminated objects. Some viruses can also be transmitted by sexual contact or congenitally (passed from a mother to her fetus). Viruses cause a wide range of illnesses, from the common cold and flu to more severe diseases like Ebola and HIV.
Viral infections are very difficult to eradicate, especially in humans. Viruses are very small and hard to detect without the aid of a microscope. They have different shapes, such as helical and icosahedral, and they can vary in size. They are also extremely resistant to most drugs and cannot be killed by heat or ultraviolet light.
When a virus enters a host cell, it hijacks the cell’s machinery to replicate itself. This process robs the host cell of its ability to produce vital macromolecular components, causing the host to die and release the viruses, or “virions.” Virions can then infect other cells in the body.
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Infection with a virus usually causes inflammation of the tissue that the virus infects. Inflammation is often followed by a fever and aches. It can affect the whole body or a specific organ, such as the brain (encephalitis or meningitis). Some viruses, such as herpes simplex and Epstein-Barr virus, can reactivate and cause symptoms years after an infection.
Viruses can infect all parts of the body, from the nose and throat to the digestive, nervous, and reproductive systems. They can be spread by coughing or sneezing, or through direct contact with an infected body part. Vaccines, which contain inactive viral particles, can prevent some viruses from causing disease.
Unlike bacteria, viruses do not need to live in the body to survive. This makes them very hard to kill. Bacterial infections are less serious than viral infections, but they can still lead to a variety of diseases, including the plague (caused by the bacterium Yersinia pestis) and smallpox (caused by the variola virus). The most common causes of bacterial disease in humans include pneumonia, encephalitis, and gastroenteritis.
