OVERVIEW
Viral infection is the process by which a virus invades an organism by a variety of routes and proliferates in a susceptible host cell. Human viruses are viruses that can infect or cause disease in humans. The essence of viral infection is the process of interaction between the virus and the organism, and between the virus and susceptible cells. Viral infections often produce injuries or viral diseases of varying severity depending on the type of virus and the state of the organism. Virus pathogenicity is from the invasion of the host, infected cells to start, the pathogenic effect is manifested in the overall human and cellular aspects.
Virus transmission pathway
1. Horizontal transmission
Horizontal transmission refers to the transmission of the virus among different individuals in the population, including the transmission of the virus from animals to humans. Common horizontal transmission methods include the following.
(1) Transmission through the respiratory tract: viruses are infected by inhalation through the air, droplets, etc., such as influenza virus and rubella virus.
(2) Transmission through the digestive tract: Viruses contaminate food and water sources and become infected through oral ingestion. Such as hepatitis A virus, polio virus, etc..
(3) Transmission through the genitourinary tract: infection by direct sexual contact, such as human immunodeficiency virus, herpes simplex virus, etc.
(4) Transmission through skin wounds: infection by direct contact with viruses through insect bites, animal bites or skin wounds. For example, epidemic encephalitis B virus, rabies virus and so on.
(5) Transmission through blood: Infection caused by blood transfusion or blood products, including injection and organ transplantation, such as hepatitis B virus and human immunodeficiency virus.
2. Vertical transmission
The way the virus is transmitted from mother to fetus or newborn via placenta, birth canal and breastfeeding is called vertical transmission. Viruses that can be transmitted vertically include rubella virus, human immunodeficiency virus and hepatitis B virus.
Types of viral infections
Viral infections can manifest themselves in different clinical types. Based on the presence or absence of symptoms, it can be categorized into overt infection and latent infection; based on the duration of viral retention time and duration of symptoms, it can be categorized into acute infection and persistent infection.
1. Hidden infection
Due to the small number of viruses invading the body, weak virulence or strong resistance of the body, the virus proliferates in the host cells, but the body does not show obvious clinical symptoms, which is called latent infection. Occult infection allows the body to obtain specific immunity to the virus and protects the body from being re-infected by the virus. Although hidden infection does not show clinical symptoms, the virus still proliferates in the body and spreads the virus to the outside world, becoming an important source of infection.
2. Apparent infection
Due to the invasion of the body of a larger number of viruses, strong virulence or body resistance is weak, the virus in the host cell proliferation, the emergence of obvious clinical symptoms, known as overt infection. Visible infections are based on the duration of the infection. Divided into acute infection and persistent infection.
(1) acute infection: the virus invades the organism, its incubation period is short, the onset of acute, the course of the disease for several days to weeks, after the disease can often get specific immunity, the body can through its own immune mechanism to completely remove the virus out of the body, such as hepatitis A virus.
(2) Persistent infection: after the virus invades the body, it persists in the body for several months, years, or even decades, and the body may or may not show clinical symptoms and carry the virus for a long period of time, becoming an important source of infection. Persistent infection can be divided into three types according to the course of the disease and the different pathogenic mechanisms. ①Chronic infection: after the virus invades the body, it exists in the blood or tissues for a long time, and the body may or may not show symptoms. The virus can be detected throughout the course of the disease, such as chronic hepatitis caused by hepatitis B virus. ② latent infection: after primary infection, the viral genes are latent in certain tissues or cells of the body, but do not replicate and proliferate with infectious viruses, at this time, the body has no clinical symptoms, and will not discharge the virus to the outside of the body. Under certain conditions the virus can be activated and acute attack, and the virus can be detected, such as herpes simplex virus. Slow-onset viral infection: after dominant or hidden infection, the virus lurks in the body for a long time, and the incubation period can be as long as several months to several years, at this time the body is asymptomatic and the virus is not detected. Once the disease develops, it is subacute progressive aggravation until death, such as human immunodeficiency virus infection.
Pathogenesis
1. Direct effect of virus on host cells
Different kinds of viruses interact with host cells and can exhibit different forms. In addition to entering the non-accommodating cells to produce a staccato infection and terminate the infection, but also can be manifested as lysogenic infection, steady state infection, apoptosis, cell proliferation and transformation, the integration of viral genes and the formation of inclusion bodies.
2. Immunopathologic effects of viral infection
Tissue damage due to immunopathology is common in viral infections. Antigens that induce immunopathologic responses include, in addition to viruses, autoantigens that appear as a result of viral infection. In addition, some viruses can directly invade immune cells and destroy their immune function.
Screening methods
Viral infections are very common, and the examination of viral infections is not only used for the evaluation of clinical diseases, but also for epidemiological investigations to provide scientific basis for the prevention and treatment of viral diseases.
1. Optical microscope examination
Optical microscopy can be used to directly observe large individual viruses such as poxviruses, and can also directly examine the inclusion bodies in tissue cells infected by certain viruses.
2. Electron microscope examination
①Electron microscopy direct examination: used for direct examination of relevant virus particles from herpes fluid, feces or serum specimens, such as herpes virus, hepatitis A virus, hepatitis B virus particles. ② immunoelectron microscopy: the virus specimen is made into a suspension, add specific antibodies to mix, so that the virus particles in the specimen agglutinate into clusters, and then use electron microscopy, which can improve the detection rate.
3. Virus isolation and culture
The methods of virus isolation and culture in the laboratory include tissue cell culture, animal inoculation and chicken embryo culture.
4. Virus antigen and antibody examination
Commonly used methods include neutralization test, hemagglutination inhibition test, immunodiffusion test and fluorescein, enzyme, radionuclide and other labeling techniques.
Prevention and treatment of viral infections
At present, there is a lack of specific drug treatment for most viral infections, so artificial immunization is the most effective means of preventing viral infections.