Overview of Viruses

Viruses are among the smallest microbes, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer in length (megavirus, pandoravirus) have recently been discovered. Viruses depend completely on cells (bacterial, plant, or animal) to reproduce. Some viruses have an outer envelope consisting of protein and lipid, surrounding a protein capsid complex with genomic RNA or DNA and sometimes enzymes needed for the first steps of viral replication.

Classification of viruses is principally according to their genome sequence taking into consideration nature and structure of their genome and their method of replication, but not according to the diseases they cause (see International Committee on Taxonomy of Viruses (ICTV), 2021 release). Thus, there are DNA viruses and RNA viruses; either DNA or RNA viruses may have single or double strands of genetic material. Single-strand RNA viruses are further divided into those with (+) sense and (-) sense RNA. Positive-sense RNA viruses possess a single-stranded RNA genome that can serve as messenger RNA (mRNA) that can be directly translated to produce an amino acid sequence. Negative-sense RNA viruses possess a single-stranded negative-sense genome that first must synthesize a complementary positive-sense antigenome, which is then used to make genomic negative-sense RNA. DNA viruses typically replicate in the host cell nucleus, and RNA viruses typically replicate in the cytoplasm.

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 a germline cell is infected by a retrovirus, 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) (1 References Viruses are among the smallest microbes, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer in length (megavirus, pandoravirus) have recently... read more ). 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.

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.

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. Many viral infections are cleared by the immune system, 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

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 appears to persist in the immunologically privileged sites in the human body (eg, testes, eyes) (2 References Viruses are among the smallest microbes, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer in length (megavirus, pandoravirus) have recently... read more ).

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

Chronic viral infections are characterized by continuous, prolonged viral shedding; examples are congenital infection with rubella virus Congenital Rubella Congenital rubella is a viral infection acquired from the mother during pregnancy. Signs are multiple congenital anomalies that can result in fetal death. Diagnosis is by serology and viral... read more or with cytomegalovirus Congenital and Perinatal Cytomegalovirus Infection (CMV) Cytomegalovirus infection may be acquired prenatally or perinatally and is the most common congenital viral infection. Signs at birth, if present, are intrauterine growth restriction, prematurity... read more and persistent 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 or 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 . 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 can cause both latent and chronic infections.

Several hundred different viruses infect humans. Viruses that infect primarily humans often spread via respiratory and enteric excretions. Blood that is collected for transfusion is tested for a number of viruses (see table ). 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, SARS-CoV-2 COVID-19 COVID-19 is an acute, sometimes severe, respiratory illness caused by the novel coronavirus SARS-CoV-2. Prevention is by vaccination and infection control precautions (eg, face masks, handwashing... read more ).

Some viruses are transmitted sexually via mucosal contact, 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 . Other viruses are transmitted through transfer of blood (eg, through puncture by a contaminated needle, or transfusion), including 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 , and the following arboviruses:

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 and Epstein–Barr virus Infectious Mononucleosis Infectious mononucleosis is caused by Epstein-Barr virus (EBV, human herpesvirus type 4) and is characterized by fatigue, fever, pharyngitis, and lymphadenopathy. Fatigue may persist weeks or... read more are the viruses most predominantly transferred through transplantation of tissue. Other such viruses include

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) is defined as any virus that is transmitted to humans and/or other vertebrates by certain species of blood-feeding arthropods, mostly 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).

Variant Creutzfeldt-Jakob disease Variant CJD (vCJD) Creutzfeldt-Jakob disease (CJD) is the most common human prion disease. It occurs worldwide and has several forms and subtypes. CJD symptoms include dementia, myoclonus, and other central nervous... read more and bovine spongiform encephalopathy were initially thought to be caused by a virus and termed slow viral diseases because they have lengthy incubations (years), but they are now known to be caused by prions Overview of Prion Diseases Prion diseases are progressive, fatal, and untreatable degenerative brain disorders. Prominent types of prion diseases include Creutzfeldt-Jakob disease (CJD), the prototypic example (usually... read more ; prions are proteinaceous disease-causing agents that are not bacterial, fungal, or viral and that contain no genetic material.

(See also Types of Viral Disorders Types of Viral Disorders Categorizing viral infections by the organ system most commonly affected (eg, lungs, gastrointestinal tract, skin, liver, central nervous system, mucous membranes) can be clinically useful,... read more .)