Why Is Everyone Talking About Overview Of Viral Infections?

Most commonly, viral infections involve the nose, throat, and upper airways.
Doctors may base the diagnosis on symptoms, blood tests and cultures, or examination of infected tissues.
Antiviral drugs may interfere with the reproduction of viruses or strengthen the immune response to the viral infection.

A virus is a small infectious organism—much smaller than a fungus or bacterium—that must invade a living cell to reproduce (replicate). The virus attaches to a cell (called the host cell), enters it, and releases its DNA or RNA inside the cell. The virus’s DNA or RNA is the genetic material containing the information needed to make copies of (replicate) the virus. The virus’s genetic material takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. When it dies, the cell releases new viruses, which go on to infect other cells.
Viruses are classified as DNA viruses or RNA viruses, depending on whether they use DNA or RNA to replicate. RNA viruses include retroviruses, such as HIV (human immunodeficiency virus—see Human Immunodeficiency Virus Infection). RNA viruses, particularly retroviruses, are prone to mutate.

Some viruses do not kill the cells they infect but instead alter the cell’s functions. Sometimes the infected cell loses control over normal cell division and becomes cancerous. Some viruses, such as herpesviruses (see Herpesvirus Infection Overview) and HIV (see Human Immunodeficiency Virus Infection), leave their genetic material in the host cell, where the material remains dormant for an extended time (called latent infection). When the cell is disturbed, the virus may begin replicating again and cause disease.
Viruses usually infect one particular type of cell. For example, common cold viruses infect only cells of the upper respiratory tract. Additionally, most viruses infect only a few species of plants or animals. Some infect only people. Many viruses commonly infect infants and children (see Viral Infections in Infants and Children).
Viruses are spread (transmitted) in various ways. Some are swallowed, some are inhaled, and some are spread by the bites of insects, such as mosquitoes, certain biting flies, or ticks. Some are spread sexually (see Sexually Transmitted Diseases) or during transfusion of contaminated blood.

Many viruses that were once present in only a few parts of the world are now spreading. These viruses include chickungunya virus, Crimean-Congo hemorrhagic fever virus, Japanese encephalitis virus, Rift Valley Fever virus, West Nile virus, Ross River virus, Zika virus, and louping ill virus. These viruses are spreading partly because climate change has resulted in more areas where the mosquitoes that spread the viruses can live. Also, travelers may be infected, then return home and be bitten by a mosquito, which spreads the virus to other people. Chickungunya virus, which is spread by mosquitoes, was first identified in Africa but has recently spread to the Caribbean and Central, South, and North America. Chikungunya virus infection typically causes a fever and severe joint pain, often in the hands and feet. Infected people may also have a headache, muscle pain, joint swelling, or a rash. Most people feel better within a week.

Defenses against viruses
The body has a number of defenses against viruses. Physical barriers, such as the skin, discourage easy entry. Infected cells also make interferons, substances that can make uninfected cells more resistant to infection by many viruses.
When a virus enters the body, it triggers the body’s immune defenses. These defenses begin with white blood cells, such as lymphocytes and monocytes, which learn to attack and destroy the virus or the cells it has infected (see Biology of the Immune System:White blood cells). If the body survives the virus attack, some of the white blood cells remember the invader and are able to respond more quickly and effectively to a subsequent infection by the same virus. This response is called immunity. Immunity can also be produced by getting a vaccine (see Immunization).

Types of viral infections
Probably the most common viral infections are
Respiratory infections: Infections of the nose, throat, upper airways, and lungs
The most common respiratory infections are upper respiratory infections, which include sore throat, sinusitis, and the common cold.

Other viral respiratory infections include influenza and pneumonia.
In small children, viruses also commonly cause croup (which is inflammation of the upper and lower airways, called laryngotracheobronchitis) or lower airways (bronchiolitis—see Bronchiolitis).
Respiratory infections are more likely to cause severe symptoms in infants, older people, and people with a lung or heart disorder.

Other viruses infect other specific parts of the body:
Gastrointestinal tract: Infections of the gastrointestinal tract, such as gastroenteritis, are commonly caused by viruses, such as noroviruses and rotaviruses.

Liver: These infections result in hepatitis.
Nervous system: Some viruses, such as the rabies virus and the West Nile virus, infect the brain, causing encephalitis. Others infect the layers of tissue that cover the brain and spinal cord (meninges), causing meningitis or polio.

Skin: Viral infections that affect only the skin sometimes result in warts or other blemishes. Many viruses that affect other parts of the body, such as chickenpox, also cause a rash.
Some viruses typically affect many body systems. Such viruses include enteroviruses (such as coxsackieviruses and echoviruses) and cytomegaloviruses.

Viruses and cancer
Some viruses alter the DNA of their host cells in a way that helps cancer develop. Only a few viruses are known to cause cancer, but there may be others.

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