Viral disease

A viral disease (or viral infection) occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells.

Examples include the common cold, gastroenteritis, COVID-19, the flu, and rabies.

Structural characteristics

File:Virus size.png

Virions of some of the most common human viruses with their relative size. Nucleic acids are not to scale. SARS stands for SARS-CoV-1 or COVID-19, variola viruses for smallpox.

Basic structural characteristics, such as genome type, virion shape and replication site, generally share the same features among virus species within the same family.

• Double-stranded DNA families: three are non-enveloped (Adenoviridae, Papillomaviridae and Polyomaviridae) and two are enveloped (Herpesviridae and Poxviridae). All of the non-enveloped families have icosahedral capsids.
• Partly double-stranded DNA viruses: Hepadnaviridae. These viruses are enveloped.
• One family of single-stranded DNA viruses infects humans: Parvoviridae. These viruses are non-enveloped.
• Positive single-stranded RNA families: three non-enveloped (Astroviridae, Caliciviridae and Picornaviridae) and four enveloped (Coronaviridae, Flaviviridae, Retroviridae and Togaviridae). All the non-enveloped families have icosahedral nucleocapsids.
• Negative single-stranded RNA families: Arenaviridae, Bunyaviridae, Filoviridae, Orthomyxoviridae, Paramyxoviridae and Rhabdoviridae. All are enveloped with helical nucleocapsids.
• Double-stranded RNA genome: Reoviridae.
• The Hepatitis D virus has not yet been assigned to a family, but is clearly distinct from the other families infecting humans.
• Viruses known to infect humans that have not been associated with disease: the family Anelloviridae and the genus Dependovirus. Both of these taxa are non-enveloped single-stranded DNA viruses.

Pragmatic rules

Human-infecting virus families offer rules that may assist physicians and medical microbiologists/virologists.

As a general rule, DNA viruses replicate within the cell nucleus while RNA viruses replicate within the cytoplasm. Exceptions are known to this rule: poxviruses replicate within the cytoplasm and orthomyxoviruses and hepatitis D virus (RNA viruses) replicate within the nucleus.

• Segmented genomes: Bunyaviridae, Orthomyxoviridae, Arenaviridae, and Reoviridae (acronym BOAR). All are RNA viruses.
• Viruses transmitted almost exclusively by arthropods: Bunyavirus, Flavivirus, and Togavirus. Some Reoviruses are transmitted from arthropod vectors. All are RNA viruses.
• One family of enveloped viruses causes gastroenteritis (Coronaviridae). All other viruses associated with gastroenteritis are non-enveloped.

Baltimore group

This group of analysts defined multiple categories of virus. Groups:

• I - dsDNA
• II - ssDNA
• III - dsRNA
• IV - positive-sense ssRNA
• V - negative-sense ssRNA
• VI - ssRNA-RT
• VII - dsDNA-RT

Clinically important virus families and species with characteristics

Family	Baltimore group	Important species	Envelopment
Adenoviridae	I	Adenovirus	N
Herpesviridae	I	Herpes simplex, type 1, Herpes simplex, type 2, Varicella-zoster virus, Epstein–Barr virus, Human cytomegalovirus, Human herpesvirus, type 8	Y
Papillomaviridae	I	Human papillomavirus	N
Polyomaviridae	I	BK virus, JC virus	N
Poxviridae	I	Smallpox	Y
Parvoviridae	II	Parvovirus B19	N
Reoviridae	III	Rotavirus, Orbivirus, Coltivirus, Banna virus	N
Astroviridae	IV	Human astrovirus	N
Caliciviridae	IV	Norwalk virus	N
Coronaviridae	IV	Human coronavirus 229E, Human coronavirus NL63, Human coronavirus OC43, Human coronavirus HKU1, Middle East respiratory syndrome-related coronavirus, Severe acute respiratory syndrome coronavirus, Severe acute respiratory syndrome coronavirus 2	Y
Flaviviridae	IV	Hepatitis C virus, yellow fever virus, dengue virus, West Nile virus, TBE virus, Zika virus	Y
Hepeviridae	IV	Hepatitis E virus	N
Matonaviridae	IV	Rubella virus	Y
Picornaviridae	IV	coxsackievirus, hepatitis A virus, poliovirus, rhinovirus	N
Arenaviridae	V	Lassa virus	Y
Bunyaviridae	V	Crimean-Congo hemorrhagic fever virus, Hantaan virus	Y
Filoviridae	V	Ebola virus, Marburg virus	Y
Orthomyxoviridae	V	Influenza virus	Y
Paramyxoviridae	V	Measles virus, Mumps virus, Parainfluenza virus	Y
Pneumoviridae	V	Respiratory syncytial virus	Y
Rhabdoviridae	V	Rabies virus	Y
Unassigned	V	Hepatitis D	Y
Retroviridae	VI	HIV	Y
Hepadnaviridae	VII	Hepatitis B virus	Y

Clinical characteristics

The clinical characteristics of viruses may differ substantially among species within the same family:

Type	Family	Transmission	Diseases	Treatment	Prevention
Adenovirus	Adenoviridae	droplet contact fecal-oral venereal direct eye contact	gastroenteritis keratoconjunctivitis pharyngitis pharyngoconjunctival fever	None	Adenovirus vaccine hand washing covering mouth when coughing or sneezing avoiding close contact with the sick
Coxsackievirus	Picornaviridae	fecal-oral respiratory droplet contact	Hand, foot and mouth disease pleurodynia aseptic meningitis pericarditis myocarditis	None	hand washing covering mouth when coughing/sneezing avoiding contaminated food/water improved sanitation
Cytomegalovirus	Herpesviridae	vertical transmission bodily fluids	infectious mononucleosis Cytomegalic inclusion disease Premature birth liver, lung and spleen diseases in the newborn Small size at birth Small head size congenital seizures in the newborn	ganciclovir cidofovir foscarnet	hand washing avoid sharing food and drinks with others safe sex
Epstein–Barr virus	Herpesviridae	saliva	infectious mononucleosis Burkitt's lymphoma Hodgkin's lymphoma nasopharyngeal carcinoma	None	avoiding close contact with the sick
Hepatitis A virus	Picornaviridae	fecal-oral	acute hepatitis	Immunoglobulin (post-exposure prophylaxis)	Hepatitis A vaccine avoiding contaminated food/water improved sanitation
Hepatitis B virus	Hepadnaviridae	bodily fluids Vertical and sexual	acute hepatitis chronic hepatitis hepatic cirrhosis hepatocellular carcinoma	Lamivudine immunoglobulin Adefovir Entecavir Pegylated interferon alfa-2	Hepatitis B vaccine immunoglobulin (perinatal and post-exposure prophylaxis) avoiding shared needles/syringes safe sex
Hepatitis C virus	Flaviviridae	blood sexual contact	acute hepatitis chronic hepatitis hepatic cirrhosis hepatocellular carcinoma	Ribavirin Pegylated interferon alfa-2	avoiding shared needles/syringes safe sex
Herpes simplex virus, type 1	Herpesviridae	direct contact saliva	herpes labialis, cold sores – can recur by latency gingivostomatitis in children tonsillitis & pharyngitis in adults keratoconjunctivitis	acyclovir famciclovir foscarnet penciclovir	avoiding close contact with lesions safe sex
Herpes simplex virus, type 2	Herpesviridae	sexual contact vertical transmission	Skin vesicles, mucosal ulcers, Oral and/or genital Can be latent Aseptic meningitis	acyclovir famciclovir foscarnet penciclovir cidofovir	avoiding close contact with lesions safe sex
HIV	Retroviridae	sexual contact blood breast milk vertical transmission	AIDS	HAART, such as protease inhibitors and reverse-transcriptase inhibitors	zidovudine (perinatally) blood product screening safe sex avoiding shared needles/syringes
Human coronavirus 229E (HCoV-229E)	Coronaviridae	droplet contact fomites	common cold pneumonia bronchiolitis
Human coronavirus NL63 (HCoV-NL63)	Coronaviridae	droplet contact	common cold rhinitis bronchitis bronchiolitis pneumonia croup
Human coronavirus OC43 (HCoV-OC43)	Coronaviridae		common cold pneumonia
Human coronavirus HKU1 (HCoV-HKU1)	Coronaviridae		common cold pneumonia bronchiolitis
Human herpesvirus, type 8	Herpesviridae	Saliva Sexual	Kaposi sarcoma multicentric Castleman disease primary effusion lymphoma	many in evaluation-stage	avoid close contact with lesions safe sex
Human papillomavirus	Papillomaviridae	direct contact sexual contact vertical transmission	(common, flat, plantar and anogenital warts, laryngeal papillomas, epidermodysplasia verruciformis) Malignancies for some species (cervical carcinoma, squamous cell carcinomas)	liquid nitrogen laser vaporization cytotoxic chemicals interferon cidofovir	HPV vaccine avoiding close contact with lesions safe sex
Influenza virus	Orthomyxoviridae	droplet contact	influenza (Reye syndrome)	amantadine rimantadine zanamivir oseltamivir	influenza vaccine amantadine rimantadine hand washing covering mouth when coughing/sneezing avoiding close contact with the sick
Measles virus	Paramyxoviridae	droplet contact	measles postinfectious encephalomyelitis	None	MMR vaccine quarantining the sick avoiding contact with the sick
Middle East respiratory syndrome-related coronavirus (MERS-CoV)	Coronaviridae	close human contact	Middle East respiratory syndrome (MERS)
Mumps virus	Paramyxoviridae	droplet contact	mumps	None	MMR vaccine avoiding close contact with the sick
Parainfluenza virus	Paramyxoviridae	droplet contact	croup pneumonia bronchiolitis common cold	None	hand washing covering mouth when coughing/sneezing
Poliovirus	Picornaviridae	fecal-oral	poliomyelitis	None	Polio vaccine avoiding contaminated food and water improved sanitation
Rabies virus	Rhabdoviridae	animal bite droplet contact	rabies (fatal encephalitis)	Post-exposure prophylaxis	rabies vaccine avoiding rabid animals
Respiratory syncytial virus	Pneumoviridae	droplet contact hand to mouth	bronchiolitis pneumonia influenza-like syndrome severe bronchiolitis with pneumonia	(ribavirin)	hand washing avoiding close contact with the sick palivizumab in high risk individuals covering mouth when coughing/sneezing
Rubella virus	Togaviridae	Respiratory droplet contact	congenital rubella German measles	None	MMR vaccine avoiding close contact with the sick
Severe acute respiratory syndrome coronavirus (SARS-CoV)	Coronaviridae	droplet contact	severe acute respiratory syndrome (SARS)
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)	Coronaviridae	droplet contact	coronavirus disease 2019 (COVID-19)	Molnupiravir, Nirmatrelvir/ritonavir	hand washing covering mouth when coughing or sneezing social distancing
Varicella-zoster virus	Herpesviridae	droplet contact direct contact	chickenpox herpes zoster Congenital varicella syndrome	Varicella: acyclovir famciclovir valacyclovir Zoster: acyclovir famciclovir	Varicella: varicella vaccine varicella-zoster immunoglobulin avoiding close contact with the sick Zoster: vaccine varicella-zoster immunoglobulin

See also

• List of latent human viral infections
• Pathogenic bacteria