Humans have been fighting viruses long before our species had even evolved into its modern form. The latest virus to threaten public health globally is the coronavirus (COVID-19) which is spreading very easily and sustainably between people. Viruses cause many human diseases. Of those viruses that cause diseases, some cause short-term (acute) diseases and others recurring or long-term (chronic) diseases. Not all viral diseases are contagious. But many of them are. Common examples of contagious viral diseases include the flu, HIV, and herpes.
For a new viral disease to emerge, the causative virus must infect and successfully invade its host, bypassing the complex and sophisticated antiviral defenses that have evolved in all animals. Three major steps have been distinguished in the processes of viral disease emergence or re-emergence: (1) introduction of a viral pathogen into a new host species, (2) establishment of the pathogen in the new host, and (3) dissemination of the pathogen among a large number of individuals of the new host species to bring about outbreaks, epidemics or pandemics.
Many different elements can contribute to the emergence of a new virus disease; these include virologic determinants [such as mutation, recombination, reassortment (drift and shift), natural selection, fitness adaptation, evolutionary progressional], natural influences (such as ecologic, environmental and zoonotic influences) and factors pertaining to human activity.
Fig.1 Graphical abstract showing the common pathways of pathogen evolution in pandemic outbreaks. (Priyadarsini, 2020)
There are many types of viruses that cause a wide variety of viral diseases and symptoms. The organ system most commonly affected by viral infections include lungs, gastrointestinal tract, skin, liver, central nervous system, mucous membrane. Diseases caused by Viral infections include:
Respiratory infections mainly occur due to influenza viruses (A and B), respiratory syncytial virus A and B, rhinovirus, coronavirus, and adenovirus. From 2019 until now, COVID-19 emerged and has spread in a worldwide pandemic that has killed millions.
Viral gastroenteritis is an inflammation, swelling, and irritation of the inside lining of the gastrointestinal tract. Several types of viruses cause viral gastroenteritis, commonly called the stomach flu. The illness is most commonly caused by norovirus or rotavirus.
Viruses cause a wide array of skin infections. Human papillomavirus (HPV) is well recognized as the causative agent of common warts and condyloma acuminatum. The varicella virus causes chickenpox and shingles. Herpes simplex is one of several types of herpesviruses. Here are two types of herpes simplex virus (HSV): HSV-1, which is the usual oral herpes; HSV-2, which is the usual cause of genital herpes.
At least 5 specific viruses (hepatitis A, B, C, D, and E viruses) can cause hepatitis. Following exposure to any of these hepatitis viruses, two clinical outcomes are possible: acute or chronic hepatitis infection.
Hemorrhagic fevers affect multiple organ systems in the body and may be accompanied by hemorrhage, or bleeding. They may be caused by five distinct families of RNA viruses: the families Filoviridae, Flaviviridae, Rhabdoviridae, Arenaviridae, and Hantaviridae.
Neurological diseases may be caused directly by infection of the central nervous system (CNS) by specific pathogens or indirectly, through the host response to the infection. Viruses may cause meningitis, encephalitis, meningoencephalomyelitis, encephalitis, as well as strokes. For example, encephalitis and meningitis result from enteroviruses and HSV, as well as West Nile Virus.
Some viruses cause multisystem diseases. COVID-19, for instance, is not limited to local pneumonia but rather represents a multisystem illness with the involvement of different organs and potential for systemic complications. Other viruses involved in multisystem diseases are listed as follows.
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