Viruses are composed of two or three components: viruses all contain genetic material (RNA or DNA, and prions composed only of protein are not viruses); All viruses also have capsids formed by protein, which are used to wrap and protect genetic material. In addition, some viruses can form lipid envelopes when they reach the cell surface. Viruses vary in shape, from simple spirals and icosahedrons to complex structures. Virus particles are about 1% the size of bacteria. The origin of the virus is still unclear. Different viruses may originate from different mechanisms: some viruses may originate from plasmids (a circular DNA that can replicate within cells and transfer between cells), while others may originate from bacteria.
Viruses spread in various ways, and different types of viruses adopt different methods. For example, plant viruses can spread from plant to plant through insects that feed on plant juice, such as aphids; Animal viruses can be spread by mosquito bites. These organisms carrying viruses are called "vectors". Influenza viruses can be spread by coughing and sneezing; Norovirus can be transmitted through hand, foot and mouth, that is, through contact with infected hands, food and water; Rotavirus is often transmitted directly through contact with infected children; In addition, HIV can be transmitted through sexual contact.
Not all viruses will cause disease, because the replication of many viruses will not cause obvious damage to infected organs. Some viruses, such as HIV, can coexist with the human body for a long time and remain infectious without being affected by the host immune system, that is, "the virus persists." But under normal circumstances, virus infection can trigger an immune response and destroy the invading virus. These immune responses can be produced by vaccination, so that vaccinated people or animals are immune to the corresponding viruses for life. Microorganisms such as bacteria also have mechanisms to resist virus infection, such as restrictive modification systems. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat viral infections.
Viruses and human diseases
There are many kinds of human diseases caused by viruses. Common diseases such as cold, flu and chickenpox have been identified, as well as serious diseases such as smallpox, AIDS, SARS and bird flu. Other diseases may be caused by viruses; For example, human herpesvirus 6 may be related to some nervous system diseases, such as multiple sclerosis and chronic fatigue syndrome. In addition, Borna virus, which was originally considered as the pathogenic factor of equine nervous system diseases, has been found to cause human mental diseases. The ability of virulence to cause diseases is called virulence.
Different viruses have different pathogenic mechanisms, mainly depending on the types of viruses. At the cellular level, the main destructive function of virus is to cause cell lysis and lead to cell death. In multicellular organisms, once enough cells die in the body, it will affect the health of the body. Although the virus can cause disease, it can also exist harmlessly in the body. For example, the herpes simplex virus that can cause cold sores can remain dormant in the human body; This state is also called "incubation period", which is also the characteristic of all herpes viruses (including Albers Barr virus which can cause glandular fever and varicella-zoster virus which can cause chickenpox). Chickenpox, a herpes zoster virus, enters a latent state and can cause herpes zoster after awakening.
Some viruses can cause chronic infection and can replicate in the body without being affected by the host's defense system. Such viruses include hepatitis B virus and hepatitis C virus. People with chronic infections are virus carriers, because they are equivalent to storing viruses that remain infectious. When the proportion of carriers in the population is high, the disease will develop into an epidemic.
epidemiology
Virus epidemiology is a branch of modern medicine, which mainly studies people-oriented virus transmission routes and virus infection control methods. The virus can spread vertically, such as from mother to baby, or it can spread from one person to another in horizontal transmission. Examples of vertical transmission include hepatitis B virus and HIV, and babies will be infected with the virus from their mothers at birth; Another rare example is varicella-zoster virus, which can only cause mild infection, but it is fatal to the fetus or newborn baby. Horizontal transmission is the most common mode of virus transmission between people. The transmission routes of the virus include: blood exchange or sexual behavior, such as HIV, hepatitis B and hepatitis C virus; Oral saliva exchange, such as Albers-Barr virus; Food or drinking water containing viruses, such as norovirus; Inhalation of viruses in aerosol form, such as influenza virus; Mosquitoes are used as carriers and injected into human body through mosquito bites, such as dengue virus. The spread speed of virus infection depends on many factors, including population density, the number of susceptible individuals (such as lack of immunity to the virus), medical care and weather conditions.
Epidemiological research is used to cut off the infection chain of virus in people in time when viral diseases break out. Prevention and control measures are ways to control the spread of viruses. Finding the source of the virus and identifying the virus is an important link. Once the virus is confirmed, its spread can be blocked by vaccination. When there is no vaccine available, improving environmental sanitation and taking disinfection measures can also effectively prevent the spread of the virus. Usually infected people need to be isolated from other healthy people, while those who have been exposed to the virus need to be isolated. Most human and animal virus infections have a certain incubation period, during which there will be no signs and symptoms of illness. The incubation period of viral diseases varies from a few days to several weeks, but the incubation period of most viral infections has been determined. After the incubation period, there will be an "infection period", that is, infected individuals can transmit the virus to other individuals during this period. It is also very important to know the length of infection period and incubation period and control the outbreak of diseases. When the outbreak of a disease leads to an unusually high proportion of patients in a group, community or region, the disease is called an epidemic; If the disease spreads all over the world, it is called a plague.
Epidemics and plagues
Due to the spread of infectious diseases brought by European colonists, especially smallpox, the number of native Americans has been greatly reduced. There is no detailed data showing how many aborigines died of exotic diseases after Columbus arrived in America, but it is estimated to be close to 70% of the total population. This destructive influence helped the European colonists to drive and conquer the aborigines to a great extent.
Plague is a worldwide epidemic. 19 18 influenza outbreak, commonly known as Spanish influenza, was a large-scale epidemic with the highest level (level 5), which was caused by a special severe fatal influenza A virus; People infected with this flu are usually healthy young people, while most other flu cases occur in teenagers, the elderly or the infirm. The duration of Spanish influenza is 19 18 to 19 19. In the past, it was estimated that the death toll was 40 million to 50 million, but the latest research results show that this plague may cause 1 100 million deaths, accounting for 5% of the global population.
Most Marburg virus researchers believe that AIDS originated in sub-Saharan Africa in the 20th century. Now, AIDS has developed into a plague, and about 38,600 people around the world are infected with AIDS. UNAIDS and WHO estimate that since the first case of AIDS was discovered on June 5, more than 25 million people have died of AIDS, making AIDS one of the most devastating plagues in history. In 2007, there were 2.7 million new AIDS patients and 2 million people died of AIDS.
Several highly lethal virus strains are members of Filoviridae. Filovirus is a fibrous virus, which can cause hemorrhagic fever. Its members include Ebola virus and Marburg virus. In April 2005, when a disease broke out in Angola, Marburg virus attracted worldwide attention. This epidemic lasted from 10 in 2004 to 2005, and it was also the most serious plague caused by hemorrhagic fever.
cancer
Viruses are one of the causes of cancer. The main viruses related to human cancer are human papillomavirus, hepatitis B virus, Albers-Barr virus and human T-lymphocyte virus. Hepatitis virus can induce chronic viral infection and lead to liver cancer. Human T-lymphotropic virus can cause tropical spastic paraplegia and adult T-cell leukemia. Human papillomavirus is the cause of cervical cancer, skin cancer, hilar cancer and penile cancer. In herpesviridae, Kaposi's sarcoma-associated herpesvirus can cause Kaposi's sarcoma and body cavity lymphoma. Albers-Barr virus can cause Burkitt's lymphoma, Hodgkin's lymphoma, B-lymphoproliferative diseases and nasopharyngeal carcinoma.