The development of the disease—the pathogenesis—has to do with many other things beside the replication of the virus. Do variants of original viruses mix with other variants to create all new virus variants? Each virus genome is alone, but you can imagine situations where you could have two viruses co-infecting the same cell, and in those cases, they might be able to compensate for each other. It there are two genomes that infect the same cell—just like our genomes are combined during the division of the stem cells—they could recombine into a fully fixed genome as it comes out.
Those events might be quite rare, but because the virus replicates in an exponential way, even a rare event has a certain probability of occurring. Do vaccines for viruses need to be updated when variants arise? Is that true with all viruses? Some people are trying to develop a universal influenza vaccine, to try to target the antibody generation toward a part of the molecule that cannot change without making the molecule not work anymore.
But I think the evidence is that the variants are not escaping the vaccine dramatically. And of course, that also depends how quickly we can get this vaccination round to complete—how many different variants are going to emerge by the time most people are vaccinated. The coronavirus definitely is not going to be like that. But if it does become endemic and it circulates in the population all the time, then there is a chance that a slightly different virus might emerge and we may need a booster.
Another question is, as people become immune thanks to vaccinations, is that going to be a strong pressure on the virus? Variants may emerge because people are immune to the old virus. Taylor McNeil can be reached at taylor. Skip to main content. While these microbes have a dizzying array of functions and health effects, the structure of a virus is surprisingly simple.
Some are additionally enveloped in a soft, lipid wrapping. These tiny virus packages are just tens to a few hundreds of nanometers across.
This makes them smaller than most bacteria, which can be a small as roughly a tenth the size of a human blood cell. Such a tiny size means that you can't even spot most viruses with a light microscope.
The one exception, a group known as giant viruses , has members with astonishingly large genomes. These mega-viruses are hundreds of times larger than most, with capsids that span roughly to nanometers across and full viral forms that can measure up to nanometers across. Due to their simple structure, viruses cannot move or even reproduce without the help of an unwitting host cell. But when it finds a host, a virus can multiply and spread rapidly. To identify the correct host, viruses have evolved receptors on their surfaces that match up with those of their ideal target cell, letting the virus get its genetic material inside and hijack its host's cellular machinery to help it reproduce by multiplying the virus' genetic material and proteins.
Using that strategy, the minute marauders have flourished and evolved in step with their hosts. By one estimate , at least , different viruses can infect mammals alone, and even this massive number may be on the low side.
This viral army can cause symptoms as mild as a cough or as deadly as internal bleeding. Some viruses may even cause the runaway cellular growth that is the root of cancer, as is thought to be the case with human papillomavirus and cervical cancer.
Inside their cellular hosts, viruses can create an enormous number of copies and spread the infection to other cells. For example, if you get the flu, your body will be riddled with some hundred trillion viruses in just a few days —more than 10, times the number of people on Earth. How viruses spread from person to person depends on the type.
Many hitch a ride in the mist of droplets that flies from your mouth every time you cough or sneeze. A variety of factors can influence how fast these airborne viruses can spread. Flu, for one, seems to survive longer in cool, dry environments , which may be the source of its common winter spread. But in tropical regions, high humidity seems to help the flu jump from person to person.
Other viruses spread most easily through contact with other bodily fluids. For example, Ebola virus spreads from contact with infected blood, feces, or vomit. Unlike many other viruses, scientists think Ebola cannot spread through the air after people with the virus cough or sneeze. Still other viruses travel through an intermediary, like a mosquito, which then infects people by biting them. One example of these so-called mosquito-born diseases is dengue, which causes a potentially deadly flu-like infection.
The risk of dengue has risen in recent years, currently threatening roughly half of the global population, according to the World Health Organization. British physician Edward Jenner even discovered the principle of inoculation in the late eighteenth century, after he observed that people who contracted the mild cowpox disease were generally immune to the deadlier smallpox disease.
By the late nineteenth century, scientists knew that some agent was causing a disease of tobacco plants, but would not grow on an artificial medium like bacteria and was too small to be seen through a light microscope. Advances in live cell culture and microscopy in the twentieth century eventually allowed scientists to identify viruses.
Advances in genetics dramatically improved the identification process. Capsid - The capsid is the protein shell that encloses the nucleic acid; with its enclosed nucleic acid, it is called the nucleocapsid.
This shell is composed of protein organized in subunits known as capsomers. They are closely associated with the nucleic acid and reflect its configuration, either a rod-shaped helix or a polygon-shaped sphere. The capsid has three functions: 1 it protects the nucleic acid from digestion by enzymes, 2 contains special sites on its surface that allow the virion to attach to a host cell, and 3 provides proteins that enable the virion to penetrate the host cell membrane and, in some cases, to inject the infectious nucleic acid into the cell's cytoplasm.
Under the right conditions, viral RNA in a liquid suspension of protein molecules will self-assemble a capsid to become a functional and infectious virus. Envelope - Many types of virus have a glycoprotein envelope surrounding the nucleocapsid. The envelope is composed of two lipid layers interspersed with protein molecules lipoprotein bilayer and may contain material from the membrane of a host cell as well as that of viral origin.
The virus obtains the lipid molecules from the cell membrane during the viral budding process. However, the virus replaces the proteins in the cell membrane with its own proteins, creating a hybrid structure of cell-derived lipids and virus-derived proteins. Many viruses also develop spikes made of glycoprotein on their envelopes that help them to attach to specific cell surfaces. Nucleic Acid - Just as in cells, the nucleic acid of each virus encodes the genetic information for the synthesis of all proteins.
While the double-stranded DNA is responsible for this in prokaryotic and eukaryotic cells, only a few groups of viruses use DNA. Most viruses maintain all their genetic information with the single-stranded RNA. There are two types of RNA-based viruses.
In most, the genomic RNA is termed a plus strand because it acts as messenger RNA for direct synthesis translation of viral protein. A few, however, have negative strands of RNA.
In these cases, the virion has an enzyme, called RNA-dependent RNA polymerase transcriptase , which must first catalyze the production of complementary messenger RNA from the virion genomic RNA before viral protein synthesis can occur. The Influenza Flu Virus - Next to the common cold, influenza or "the flu" is perhaps the most familiar respiratory infection in the world. In the United States alone, approximately 25 to 50 million people contract influenza each year.
The symptoms of the flu are similar to those of the common cold, but tend to be more severe. Fever, headache, fatigue, muscle weakness and pain, sore throat, dry cough, and a runny or stuffy nose are common and may develop rapidly.
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