# 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 are the common cold, gastroenteritis,corona,flu,pneumonia.

## Structural characteristics

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

### 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 |