Viruses are a type of particles that potentially cause various diseases, even malignant cancers. It is too small to be seen without a microscope. As parasitical microbes, viruses can not self-replicate and only multiply within a host. Without a host cell, viruses cannot carry out their life-sustaining functions or reproduce. A single virus particle is called a virion, which is composed of the viral genome, contained within a protein shell termed a capsid. There is an envelope enclosing the protein coat in some viruses. Viruses are divided into DNA viruses or RNA viruses, depending on their genetic materials are DNA or RNA.
A virus infection refers to the process by which a virus attaching on the cell membrane of the host releases its genetic materials into the host cell through membrane fusion, subsequently hijacks the cellular processes and reprograms the cells to produce new viruses until the cells burst and die. Some cells even mutate into tumorous ones.
There are two viral entry pathways. First, viruses entering the host ( human, plants, animals, even bacteria) must attach to the host cell plasma membrane, followed by membrane fusion. Some viruses can fuse directly to the plasma membrane through the binding of the viral fusion protein to a special receptor on the surface of the cell membrane. Whereas, others are entirely swallowed into an endosome and then fuse their envelope with the membrane of the engulfing internal organelle. The viral genome passes through a fusion pore into the cytosol in either fusion way, and infection is initiated.
Viruses are not beneficial to humans at all. Nevertheless, most bacteria are harmless. Some species of bacteria are helpful to keep us healthy, and they are called good bacteria. For example, some bacteria in the gastrointestinal tract help digest food. The good bacteria are distributed on our skin, in our airways, and our digestive system. And they are the first line of defense against foreign pathogens that can cause infection and other problems. Fewer than 1% of bacteria cause diseases in people.
A virus infection can cause a series of symptoms from asymptomatic to severe disease. Different viral infections cause distinct symptoms. Common diseases such as influenza caused by influenza viruses are characterized by fever, headache, and muscle aches. Mononucleosis caused by EB virus infection manifests as fever, rash, hepatosplenomegaly, and abnormal liver function. Measles virus infection is featured by repeated fever, cough, rash, and conjunctival congestion. The main symptoms of SARS-CoV-2 infection are fever, fatigue, and dry cough. Approximately half of the COVID-19 patients experienced dyspnea after one week, and the severe cases progressed rapidly to acute respiratory distress syndrome, septic shock, difficult to correct metabolic acidosis, and coagulopathy.
The symptoms caused by different viral infections are different and may vary from person to person. Doctors may base the diagnosis on relevant examinations such as blood tests and cultures, or examination of infected tissues, together with clinical symptoms.
Viral infections commonly involve the skin, throat, nose, upper airways, or systems such as the nervous, gastrointestinal, and reproduction. According to infectious sites, virus infections could be divided into respiratory viral infections, viral skin infections, foodborne viral infections, and sexually transmitted viral infections, and so on.
Respiratory viral infections affect the nose, throat, and lungs. Such viruses, including seasonal influenza, rhinovirus, respiratory syncytial virus (RSV), etc. are transmitted by inhaling droplets containing virus particles. The upper respiratory infections are the most frequent respiratory infections, which include sore throat, sinusitis, and the common cold.
Viral skin infections often trigger a rash, and their symptoms range from mild to severe. For instance, varicella-zoster virus (VZV) leads to itchy, oozing blisters, fatigue, and high fever characteristic of chickenpox. Herpes simplex virus-1 (HSV-1) is frequent to cause cold sores. People at any age may catch the molluscum contagiosum, but children between the ages of 1 and 10 are more vulnerable to this virus.
Foodborne viral infections are caused by viral foodborne pathogens, including norovirus (NoV), hepatitis A, and rotavirus. Hepatitis A mainly attacks the liver, resulting in yellow skin, nausea, diarrhea, and vomiting. Norovirus once caused a severe gastrointestinal infection outbreak on cruise ships. Rotavirus can invade any age people, and babies and young children are a vulnerable group.
Sexually transmitted viral infections are predominantly spread through contact with bodily fluids. Several types of virus are also transmitted through the blood-borne transmission. For example, human immunodeficiency virus (HIV) mainly attacks the immune system, thus weakening the body's ability to defend against disease and infection, causing acquired immune deficiency syndrome (AIDS). Hepatitis B induces inflammation in the liver.
A virus infection typically lasts 7 to 10 days. Some virus infections such as chronic hepatitis, which is caused by the hepatitis B virus and hepatitis C virus, can persist in the human body for years or decades, or even for the whole life. The current SARS-CoV-2 exists an incubation period in the human body. Some people infected with the SARS-CoV-2 may not develop symptoms until 7 to 14 days or even a month later. Some cases even did not develop any symptoms at all, but there existed SARS-CoV-2 in their body.
Common modes of viral transmission include oral and nasal inhalation, insect bites, sharing contaminated needles, mother-to-child transmission, and sexual contact. Some viruses such as the SARS- CoV-2 in late 2019 even can enter the body through the eyes. Therefore, medical workers who come into close contact with COVID-19 patients must wear protective clothing, masks, and goggles. And some viral skin infections are spread via saliva by kissing or sharing tableware with a contracted one. Contaminated food and water are other potential sources of viral infection.
Viral infections are usually contagious. But viral infections are all spread in certain ways. So don't panic when you hear about a virus or contact with the patients contracted by a certain virus. For example, the dreaded HIV, other than mother-to-child transmission, needle sharing, and sexual transmission, is not transmitted through everyday contact. The influenza pandemic is partial because its early symptoms are so similar to those of the common cold that many people ignore it, and population concentration is also a factor.
Many viral infections can heal through the body's immune system other than any treatment. When the body's immune system is not strong enough to fight against these viruses, some antiviral drugs can be used to relieve the symptoms, not to cure the infection. Antiviral drugs are most effective when taken early in the onset of a virus infection or a recurrent outbreak. There have been many antiviral medications in the market, and they may be used to treat HIV, hepatitis B, hepatitis C, and influenza, etc.
Due to the lack of effective treatment for viral infection, it is particularly important to do a good job in prevention. In addition to isolating the source of infection and cutting off the route of transmission, immunization prevention is a crucial and effective measure.
Different viral infections have different treatments, but basic precautions are the same. To effectively reduce the occurrence and spread of the disease, be sure to remember to wash your hands frequently, cover your mouth and nose when sneezing or coughing, try to avoid going to densely populated places or epidemic areas, do not eat raw or dirty food, avoid contact with people with infectious diseases, etc.
Besides, vaccines can help protect you from getting many viral diseases and reduce infectious risk. With the progress of medicine, an increasing number of vaccines have been developed, such as the vaccines against the flu, hepatitis A, hepatitis B, human papillomavirus (HPV), chickenpox, herpes zoster (shingles), measles/mumps/rubella (MMR), polio, rabies, rotavirus, and so on.
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