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Overview of Viruses

By

Laura D Kramer

, PhD, Wadsworth Center, New York State Department of Health

Last full review/revision Jun 2021| Content last modified Jun 2021
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Viruses are the smallest parasites, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer long (megavirus, pandoravirus) have recently been discovered. Viruses depend completely on cells (bacterial, plant, or animal) to reproduce. Viruses have an outer cover of protein and sometimes lipid, an RNA or DNA core, and sometimes enzymes needed for the first steps of viral replication.

Viruses are classified principally according to the nature and structure of their genome and their method of replication, not according to the diseases they cause. Thus, there are DNA viruses and RNA viruses; each type may have single or double strands of genetic material. Single-strand RNA viruses are further divided into those with (+) sense and (-) sense RNA. DNA viruses typically replicate in the host cell nucleus, and RNA viruses typically replicate in the cytoplasm. However, certain single-strand, (+) sense RNA viruses termed retroviruses use a very different method of replication.

Retroviruses use reverse transcription to create a double-stranded DNA copy (a provirus) of their RNA genome, which is inserted into the genome of their host cell. Reverse transcription is accomplished using the enzyme reverse transcriptase, which the virus carries with it inside its shell. Examples of retroviruses are the human immunodeficiency viruses and the human T-cell leukemia viruses. Once the provirus is integrated into the host cell DNA, it is transcribed using typical cellular mechanisms to produce viral proteins and genetic material. If the infected cell belongs to the germline, the integrated provirus can become established as an endogenous retrovirus that is transmitted to offspring.

The sequencing of the human genome revealed that at least 1% of the human genome consists of endogenous retroviral sequences, representing past encounters with retroviruses during the course of human evolution. A few endogenous human retroviruses have remained transcriptionally active and produce functional proteins (eg, the syncytins that contribute to the structure of the human placenta). Some experts speculate that some disorders of uncertain etiology, such as multiple sclerosis, certain autoimmune disorders, and various cancers, may be caused by endogenous retroviruses.

Because RNA transcription does not involve the same error-checking mechanisms as DNA transcription, RNA viruses, particularly retroviruses, are particularly prone to mutation.

For infection to occur, the virus first attaches to the host cell at one or one of several receptor molecules on the cell surface. The viral DNA or RNA then enters the host cell and separates from the outer cover (uncoating) and replicates inside the host cell in a process that requires specific enzymes. The newly synthesized viral components then assemble into a complete virus particle. The host cell typically dies, releasing new viruses that infect other host cells. Each step of viral replication involves different enzymes and substrates and offers an opportunity to interfere with the process of infection.

The consequences of viral infection vary considerably. Many infections cause acute illness after a brief incubation period, but some are asymptomatic or cause minor symptoms that may not be recognized except in retrospect. Many viral infections are cleared by the body’s defenses, but some remain in a latent state, and some cause chronic disease.

In latent infection, viral RNA or DNA remains in host cells but does not replicate or cause disease for a long time, sometimes for many years. Latent viral infections may be transmissible during the asymptomatic period, facilitating person-to-person spread. Sometimes a trigger (particularly immunosuppression) causes reactivation.

Common viruses that remain latent include

Some disorders are caused by viral reactivation in the central nervous system after a very long latency period. These diseases include

Several hundred different viruses infect humans. Viruses that infect primarily humans often spread via respiratory and enteric excretions. Some are transmitted sexually and through transfer of blood (eg, via transfusion [arboviruses such as chikungunya Chikungunya disease Arbovirus (arthropod-borne virus) applies to any virus that is transmitted to humans and/or other vertebrates by certain species of blood-feeding arthropods, chiefly insects (flies and mosquitoes)... read more , dengue Dengue Dengue is a mosquito-borne disease caused by a flavivirus. Dengue fever usually results in abrupt onset of high fever, headache, myalgias, arthralgias, and generalized lymphadenopathy, followed... read more , West Nile West Nile Virus West Nile virus is a flavivirus that is now the primary cause of arbovirus encephalitis in the US. Most patients have mild or no symptoms. About 1 out of 150 patients develop a severe infection... read more , Zika Zika Virus (ZV) Infections The Zika virus is a mosquito-borne flavivirus that is antigenically and structurally similar to the viruses that cause dengue, yellow fever, and West Nile virus. Zika virus infection is typically... read more , and hepatitis viruses A Hepatitis A Hepatitis A is caused by an enterically transmitted RNA virus that, in older children and adults, causes typical symptoms of viral hepatitis, including anorexia, malaise, and jaundice. Young... read more , B Hepatitis B, Chronic Hepatitis B is a common cause of chronic hepatitis. Patients may be asymptomatic or have nonspecific manifestations such as fatigue and malaise. Diagnosis is by serologic testing. Without treatment... read more , C Hepatitis C, Chronic Hepatitis C is a common cause of chronic hepatitis. It is often asymptomatic until manifestations of chronic liver disease occur. Diagnosis is confirmed by finding positive anti-HCV and positive... read more , and E Hepatitis E Hepatitis E is caused by an enterically transmitted RNA virus and causes typical symptoms of viral hepatitis, including anorexia, malaise, and jaundice. Fulminant hepatitis and death are rare... read more ], mucosal contact, or puncture by a contaminated needle) or through transplantation of tissue (predominantly cytomegalovirus [CMV] Cytomegalovirus (CMV) Infection Cytomegalovirus (CMV, human herpesvirus type 5) can cause infections that have a wide range of severity. A syndrome of infectious mononucleosis that lacks severe pharyngitis is common. Severe... read more , but also arboviruses such as Zika Zika Virus (ZV) Infections The Zika virus is a mosquito-borne flavivirus that is antigenically and structurally similar to the viruses that cause dengue, yellow fever, and West Nile virus. Zika virus infection is typically... read more , West Nile West Nile Virus West Nile virus is a flavivirus that is now the primary cause of arbovirus encephalitis in the US. Most patients have mild or no symptoms. About 1 out of 150 patients develop a severe infection... read more , and dengue Dengue Dengue is a mosquito-borne disease caused by a flavivirus. Dengue fever usually results in abrupt onset of high fever, headache, myalgias, arthralgias, and generalized lymphadenopathy, followed... read more ; lymphocytic choriomeningitis virus [LCMV] Lymphocytic Choriomeningitis Lymphocytic choriomeningitis is caused by an arenavirus. It usually causes a flu-like illness or aseptic meningitis, sometimes with rash, arthritis, orchitis, parotitis, or encephalitis. Diagnosis... read more , HIV-1 Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection results from 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ lymphocytes and impair cell-mediated immunity, increasing risk of certain... read more Human Immunodeficiency Virus (HIV) Infection , rabies Rabies Rabies is a viral encephalitis transmitted by the saliva of infected bats and certain other infected mammals. Symptoms include depression and fever, followed by agitation, excessive salivation... read more , hepatitis B [HBV] Hepatitis B, Chronic Hepatitis B is a common cause of chronic hepatitis. Patients may be asymptomatic or have nonspecific manifestations such as fatigue and malaise. Diagnosis is by serologic testing. Without treatment... read more , and herpes simplex virus [HSV] Overview of Herpesvirus Infections Eight types of herpesviruses infect humans (see Table: Herpesviruses That Infect Humans). After initial infection, all herpesviruses remain latent within specific host cells and may subsequently... read more ). Blood that is collected for transfusion is tested for a number of viruses (see table Infectious Disease Transmission Testing Infectious Disease Transmission Testing More than 21 million units of blood components are transfused yearly in the US, from about 7 million volunteer donors (1). Although transfusion is probably safer than ever, risk (and the public’s... read more ). Many viruses are transmitted via rodent or arthropod vectors, and bats have recently been identified as hosts for many mammalian viruses, including some responsible for certain serious human infections (eg, COVID-19 COVID-19 Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia. Numerous coronaviruses, first discovered in domestic poultry... read more ).

Viruses exist worldwide, but their spread is limited by inborn resistance, prior immunizing infections or vaccines, sanitary and other public health control measures, and prophylactic antiviral drugs.

Zoonotic viruses Overview of Arbovirus, Arenavirus, and Filovirus Infections Arbovirus (arthropod-borne virus) applies to any virus that is transmitted to humans and/or other vertebrates by certain species of blood-feeding arthropods, chiefly insects (flies and mosquitoes)... read more pursue their biologic cycles chiefly in animals; humans are secondary or accidental hosts. These viruses are limited to areas and environments able to support their nonhuman natural cycles of infection (vertebrates, arthropods, or both).

Reference

Viruses and cancer

Some viruses are oncogenic and predispose to certain cancers:

Diagnosis of Viral Infections

Some viral disorders can be diagnosed as follows:

Definitive laboratory diagnosis is necessary mainly when specific treatment may be helpful or when the agent may be a public health threat (eg, HIV). Typical hospital laboratories can test for some viruses, but for less common disorders (eg, rabies Rabies Rabies is a viral encephalitis transmitted by the saliva of infected bats and certain other infected mammals. Symptoms include depression and fever, followed by agitation, excessive salivation... read more , Eastern equine encephalitis, human parvovirus B19), specimens must be sent to state health laboratories or the Centers for Disease Control and Prevention.

Serologic examination during acute and convalescent stages can be sensitive and specific, but slow; with some viruses, especially flaviviruses, cross-reactions confound diagnosis. More rapid diagnosis can sometimes be made using culture, polymerase chain reaction, or viral antigen tests. Histopathology with electron (not light) microscopy can sometimes help. For specific diagnostic procedures, see Laboratory Diagnosis of Infectious Disease Introduction to Laboratory Diagnosis of Infectious Disease Laboratory tests may identify organisms directly (eg, visually, using a microscope, growing the organism in culture) or indirectly (eg, identifying antibodies to the organism). General types... read more .

Viral genomes are small; the genome of RNA viruses ranges from 3.5 kilobases (some retroviruses) to 27 kilobases (some reoviruses), and the genome of DNA viruses ranges from 5 kilobases (some parvoviruses) to 280 kilobases (some poxviruses). This manageable size together with the current advances in nucleotide sequencing technology means that partial and whole virus genome sequencing will become an essential component in epidemiologic investigations of disease outbreaks.

Treatment of Viral Infections

Antiviral drugs

Progress in the use of antiviral drugs is occurring rapidly. Antiviral chemotherapy can be directed at various phases of viral replication. It can

  • Interfere with viral particle attachment to host cell membranes or uncoating of viral nucleic acids

  • Inhibit a cellular receptor or factor required for viral replication

  • Block specific virus-coded enzymes and proteins that are produced in the host cells and that are essential for viral replication but not for normal host cell metabolism

Antiviral drugs are most often used therapeutically or prophylactically against herpesviruses Overview of Herpesvirus Infections Eight types of herpesviruses infect humans (see Table: Herpesviruses That Infect Humans). After initial infection, all herpesviruses remain latent within specific host cells and may subsequently... read more (including cytomegalovirus Cytomegalovirus (CMV) Infection Cytomegalovirus (CMV, human herpesvirus type 5) can cause infections that have a wide range of severity. A syndrome of infectious mononucleosis that lacks severe pharyngitis is common. Severe... read more ), respiratory viruses Overview of Viral Respiratory Infections Viral infections commonly affect the upper or lower respiratory tract. Although respiratory infections can be classified by the causative virus (eg, influenza), they are generally classified... read more , HIV Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection results from 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ lymphocytes and impair cell-mediated immunity, increasing risk of certain... read more Human Immunodeficiency Virus (HIV) Infection , chronic hepatitis B Hepatitis B, Chronic Hepatitis B is a common cause of chronic hepatitis. Patients may be asymptomatic or have nonspecific manifestations such as fatigue and malaise. Diagnosis is by serologic testing. Without treatment... read more , and chronic hepatitis C Hepatitis C, Chronic Hepatitis C is a common cause of chronic hepatitis. It is often asymptomatic until manifestations of chronic liver disease occur. Diagnosis is confirmed by finding positive anti-HCV and positive... read more . However, some drugs are effective against many different kinds of viruses. Some drugs active against HIV are used for other viral infections such as hepatitis B. New antiviral drugs are effective against Ebola virus Treatment Marburg and Ebola are filoviruses that cause hemorrhage, multiple organ failure, and high mortality rates. Diagnosis is with enzyme-linked immunosorbent assay, polymerase chain reaction (PCR)... read more .

Interferons

Interferons are compounds released from infected host cells in response to viral or other foreign antigens.

There are many different interferons, which have numerous effects such as blocking translation and transcription of viral RNA and stopping viral replication without disturbing normal host cell function.

Interferons are sometimes given attached to polyethylene glycol (pegylated formulations), allowing slow, sustained release of the interferon.

Viral disorders sometimes treated with interferon therapy include

Adverse effects of interferons include fever, chills, weakness, and myalgia, typically starting 7 to 12 hours after the first injection and lasting up to 12 hours. Depression, hepatitis, and, when high doses are used, bone marrow suppression are also possible.

Antibodies

Prevention of Viral Infections

Vaccines

Vaccines Immunity can be achieved Actively by using antigens (eg, vaccines, toxoids) Passively by using antibodies (eg, immune globulins, antitoxins) A toxoid is a bacterial toxin that has been modified... read more work by stimulating immunity. Viral vaccines in general use include hepatitis A Hepatitis A (HepA) Vaccine Both hepatitis A vaccines provide long-term protection against hepatitis A. For more information, see Hepatitis A Advisory Committee on Immunization Practices Vaccine Recommendations and Centers... read more , hepatitis B Hepatitis B (HepB) Vaccine The hepatitis B vaccine is 80 to 100% effective in preventing infection or clinical hepatitis B in people who complete the vaccine series. For more information, see Hepatitis B Advisory Committee... read more , human papillomavirus Human Papillomavirus (HPV) Vaccine Human papillomavirus (HPV) infection is the most common sexually transmitted disease. HPV can cause skin warts, genital warts, or certain cancers, depending on the type of HPV. Vaccines are... read more , influenza Influenza Vaccine Based on recommendations by the World Health Organization and the Centers for Disease Control and Prevention (CDC), vaccines for influenza are modified annually to include the most prevalent... read more , Japanese encephalitis, measles Measles, Mumps, and Rubella (MMR) Vaccine The measles, mumps, and rubella (MMR) vaccine effectively protects against all 3 infections. People who are given the MMR vaccine according to the US vaccination schedule are considered protected... read more , mumps Measles, Mumps, and Rubella (MMR) Vaccine The measles, mumps, and rubella (MMR) vaccine effectively protects against all 3 infections. People who are given the MMR vaccine according to the US vaccination schedule are considered protected... read more , poliomyelitis Poliomyelitis Vaccine Extensive vaccination has almost eradicated polio worldwide. But cases still occur in areas with incomplete immunization, such as sub-Saharan Africa and southern Asia. There are 3 serotypes... read more , rabies Prevention Rabies is a viral encephalitis transmitted by the saliva of infected bats and certain other infected mammals. Symptoms include depression and fever, followed by agitation, excessive salivation... read more , rotavirus Recommended Immunization Schedule for Ages 0–6 Years Vaccination follows a schedule recommended by the Centers for Disease Control and Prevention (CDC), the American Academy of Pediatrics, the American Academy of Family Physicians, and the American... read more , rubella Measles, Mumps, and Rubella (MMR) Vaccine The measles, mumps, and rubella (MMR) vaccine effectively protects against all 3 infections. People who are given the MMR vaccine according to the US vaccination schedule are considered protected... read more , tick-borne encephalitis, varicella Varicella Vaccine Varicella vaccination provides effective protection against varicella (chickenpox). It is not known how long protection against varicella lasts. But, live-virus vaccines, like the varicella... read more , and yellow fever Yellow fever is a mosquito-borne flavivirus infection endemic in tropical South America and sub-Saharan Africa. Symptoms may include sudden onset of fever, relative bradycardia, headache, and... read more . Adenovirus and smallpox Prevention Smallpox is a highly contagious disease caused by the smallpox virus, an orthopoxvirus. It causes death in up to 30%. Natural infection has been eradicated. The main concern for outbreaks is... read more Prevention vaccines are available but used only in high-risk groups (eg, military recruits). The US Food and Drug Administration (FDA) approved a vaccine for prevention of disease caused by the Zaire Ebola virus Ebola Vaccine rVSV-ZEBOV is the only vaccine approved by the U.S. Food and Drug Administration (FDA) for prevention of Ebola virus disease. Ad26.ZEBOV/MVA-BN-Filo is a combination of two vaccines, Ad26.ZEBOV... read more that is being used in the current outbreak in the Democratic Republic of the Congo (DRC), and the DRC Ministry of Health approved the use of a second vaccine combination for use in the country outside of the outbreak zone (1 Prevention reference Viruses are the smallest parasites, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer long (megavirus, pandoravirus) have recently been discovered... read more ). Multiple vaccines have received emergency use authorization for prevention of COVID-19 COVID-19 Vaccine COVID-19 vaccines provide protection against COVID-19. COVID-19 is the disease caused by infection with the SARS-CoV-2 virus. There are multiple COVID-19 vaccines currently in use worldwide... read more caused by SARS-CoV-2 virus.

Viral diseases can be eradicated by good vaccines. Smallpox was eradicated in 1978, and the cattle plague rinderpest (caused by a virus closely related to human measles virus) was eradicated in 2011. Poliomyelitis has been eradicated from all but a few countries where logistics and religious sentiment continue to impede vaccination. Measles has been almost eradicated from some parts of the world, notably the Americas, but because measles is highly contagious and vaccination coverage is incomplete, even in regions where it is considered eradicated, final eradication is not imminent.

The prospects for eradication of other more intractable virus infections (such as HIV) are presently uncertain.

Immune globulins

Immune globulins Passive Immunization Passive immunization involves giving Antibodies to an organism or a toxin produced by an organism Passive immunization is provided in the following circumstances: When people cannot synthesize... read more are available for passive immune prophylaxis in limited situations. They can be used preexposure (eg, for hepatitis A), postexposure (eg, for rabies, varicella, respiratory syncytial virus, hepatitis), and for treating disease (eg, eczema vaccinatum).

Protective measures

Many viral infections can be prevented by commonsense protective measures (which vary depending on the transmission mode of a given agent).

Important measures include

  • Hand washing

  • Appropriate food preparation and water treatment

  • Avoidance of contact with sick people

  • Safe-sex practices

  • Mask wearing; physical distancing when appropriate (eg, for COVID-19 prevention)

For infections with an insect vector (eg, mosquitoes, ticks), avoiding the vector is important.

Prevention reference

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