Overview of Viral Infections
People may get viruses by swallowing or inhaling them, by being bitten by insects, or through sexual contact.
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 (HIV) 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 (HIV) 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 (STDs)) 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 chikungunya 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. Chikungunya 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.
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 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).
Probably the most common viral infections are
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:
Liver: These infections result in hepatitis.
Some viruses typically affect many body systems. Such viruses include enteroviruses (such as coxsackieviruses and echoviruses) and cytomegaloviruses.
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.
Viruses and Cancer: A Link
Common viral infections (such as measles, rubella, or chickenpox) may be diagnosed based on symptoms.
For infections that occur in epidemics (such as influenza), the presence of other similar cases may help doctors identify a particular infection.
For other infections, blood tests and cultures (growing microorganisms in the laboratory from samples of blood, body fluid, or other material taken from an infected area) may be done. Blood may be tested for antibodies to viruses or for antigens (proteins on or in viruses that trigger the body’s defenses). Polymerase chain reaction (PCR) techniques may be used to make many copies of the viral genetic material, enabling doctors to rapidly and accurately identify the virus. Tests are sometimes done quickly—for instance, when the infection is a serious threat to public health or when symptoms are severe.
A sample of blood or other tissues is sometimes examined with an electron microscope, which provides high magnification with clear resolution.
There are no specific treatments for many viruses. However, many things can help relieve certain symptoms, such as the following:
Dehydration: Plenty of fluids, sometimes given by vein (intravenously)
Diarrhea: Sometimes loperamide
Fever and aches: Acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs)
Nausea and vomiting: A clear-liquid diet and sometimes an antiemetic (antinausea) drug, such as ondansetron
A rash (some): Soothing or moisturizing creams and sometimes an antihistamine for itching
A runny nose: Sometimes nasal decongestants, such as phenylephrine or phenylpropanolamine
A sore throat: Sometimes throat-numbing lozenges containing benzocaine or dyclonine
Not everyone who has these symptoms needs treatment. If symptoms are mild, it may be better to wait for them to go away on their own. Some treatments may not be appropriate for infants and young children.
Drugs that combat viral infections are called antiviral drugs. There are no effective antiviral drugs for many viral infections. However, there are several drugs for influenza (see Influenza (Flu) : Treatment), many drugs for infection by one or more herpesviruses (see Table: Some Antiviral Drugs for Herpesvirus Infections), and many new antiviral drugs for treatment of HIV (see Table: Drugs for HIV Infection) and hepatitis C (see Chronic Hepatitis: Treatment) infections.
Antiviral drugs can work by
Many antiviral drugs work by interfering with replication of viruses. Most drugs used to treat human immunodeficiency virus (HIV) infection (see Human Immunodeficiency Virus (HIV) Infection : Treatment) work this way. Because viruses are tiny and replicate inside cells using the cells' own metabolic functions, there are only a limited number of metabolic functions that antiviral drugs can target. In contrast, bacteria are relatively large organisms, commonly reproduce by themselves outside of cells, and have many metabolic functions that antibacterial drugs (antibiotics) can target. Therefore, antiviral drugs are much more difficult to develop than antibacterial drugs. Also, unlike antibiotics, which are usually effective against many different species of bacteria, most antiviral drugs are usually effective against only one (or a very few) viruses.
Antiviral drugs can be toxic to human cells. Also, viruses can develop resistance to antiviral drugs.
Other antiviral drugs strengthen the immune response to the viral infection. These drugs include several types of interferons, immunoglobulins, and vaccines:
Interferon drugs are replicas of naturally occurring substances that slow or stop viral replication.
Immune globulin is a sterilized solution of antibodies (also called immunoglobulins) collected from a group of people.
Vaccines are materials that help prevent infection by stimulating the body's natural defense mechanisms (see Immunization).
Many immune globulins and vaccines are given before exposure to a virus to prevent infection. Some immune globulins and some vaccines, such as those for rabies and hepatitis B, are also used after exposure to the virus to help prevent infection from developing or reduce the severity of infection. Immune globulins may also help treat some infections.
Most antiviral drugs can be given by mouth. Some can also be given by injection into a vein (intravenously) or muscle (intramuscularly). Some are applied as ointments, creams, or eye drops or are inhaled as a powder.
Antibiotics are not effective against viral infections, but if a person has a bacterial infection in addition to a viral infection, an antibiotic is often necessary.