Outside a cell, a virus wraps itself up into an independent particle called a virion. When a virion attaches to a suitable host cell — this depends on the protein molecules on the surfaces of the virion and the cell — it is able to penetrate the cell. The virions make their way out of the cell, usually destroying it in the process, and then head off to infect more cells. At the core of a virus particle is the genome, the long molecule made of DNA or RNA that contains the genetic instructions for reproducing the virus.
This is wrapped up in a coat made of protein molecules called a capsid, which protects the genetic material. Some viruses also have an outer envelope made of lipids, which are fatty organic molecules. Soap can dissolve this fatty envelope, leading to the destruction of the whole virus particle.
Viruses are like predators with a specific prey they can recognise and attack. Scientists categorize viruses according to various factors, including:.
Examples of viruses with an envelope include the influenza virus and HIV. Within these categories are different types of viruses. A coronavirus, for example, has a sphere-like shape and a helical capsid containing RNA. It also has an envelope with crown-like spikes on its surface. Seven coronaviruses can affect humans, but each one can change or mutate, producing many variants.
Learn more about coronaviruses here. Just as there are friendly bacteria in the intestines that are essential to gut health , humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli.
Viruses do not leave fossil remains, so they are difficult to trace through time. Scientists use molecular techniques to compare the DNA and RNA of viruses and find out more about where they come from.
Three competing theories try to explain the origin of viruses. In reality, viruses may have evolved in any of these ways. The regressive, or reduction, hypothesis suggests that viruses started as independent biological entities that became parasites. Over time, they shed genes that did not help them parasitize, and became entirely dependent on the cells they inhabit. In this way, they gained the ability to become independent and move between cells.
The virus-first hypothesis suggests that viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells on Earth appeared, billions of years ago. When a viral disease emerges, it is not always clear where it comes from. A virus exists only to reproduce. When it reproduces, particles spread to new cells and new hosts. The features of a virus affect its ability to spread.
Some viruses can remain active on an object for some time. If a person with the virus on their hands touches an item, the next person can pick up that virus by touching the same object. The object is known as a fomite. Viruses often change over time. Some of these changes are very small and do not cause concern, but others can be more significant.
Significant changes could make a virus more transmissible, as has been the case with the B. They may also help the virus evade the immune system or existing treatments. For example, doctors use several drugs in combination to treat HIV so that it is harder for the virus to develop resistance to treatment. Influenza viruses can also do so-called antigenic shift. There's no reason not to continue to go to school or work if you feel well enough.
Measles Symptoms of measles appear around 10 days after you become infected. Measles is most infectious after the first symptoms appear and before the rash develops. First symptoms of measles include: a high temperature red eyes cold-like symptoms — such as a runny nose, watery eyes, swollen eyelids and sneezing Around 2 to 4 days later, a red-brown spotty rash develops that normally fades after about a week.
Mumps Mumps causes your salivary glands to swell. Mumps is most infectious from a few days before your glands swell until a few days afterwards. Rubella german measles People with rubella should stay off school or work, and avoid contact with pregnant women where possible, for 6 days after the rash firsts develops.
Shingles You can't spread shingles to others. Shingles is infectious while the rash oozes fluid. Tonsillitis Tonsillitis itself isn't contagious, but the viruses that cause it are. The public health measures taken to stem the spread of SARS-CoV-2, which has been responsible for the deaths of more than , people in the U. This year the Southern Hemisphere essentially skipped flu season , which typically hits countries such as Australia, Chile and South Africa in May or June. Data from Australia suggest that although pandemic restrictions pushed many non-flu viruses out of circulation, a group of cold-causing pathogens known as rhinoviruses stuck around.
A similar trend could be in store for the U. Pedro Piedra, a pediatric infectious disease specialist at Baylor College of Medicine, says that although he has seen a significant decrease in many common respiratory viruses during the pandemic, he has noticed an uptick in rhinoviruses this fall.
Some virologists believe that the sheer number of viruses that cause the common cold can make it exceedingly difficult to avoid catching one: there are around different pathogens. These include four coronaviruses the group that includes SARS-CoV-2 ; four parainfluenza viruses which, despite their name, bear no relation to influenza viruses ; respiratory syncytial virus; and different rhinoviruses. Viral censuses have revealed that dozens of these rhinoviruses circulate in any one place at a given time.
The persistence of rhinoviruses during the pandemic may be the result of not only their impressive number but also their primitive nature, says Ian Mackay, a virologist at the University of Queensland in Australia. This envelope can cloak the pathogen from antibodies deployed by the human immune system, enabling it to infect cells undetected. But it can also break down after exposure to the environment or a good handwashing, rendering the virus harmless.
Rhinoviruses, on the other hand, never evolved an envelope. These so-called naked viruses, which also include the gut-distress-inducing noroviruses, are more resistant to sanitizers and disinfectants and may last longer on fingertips and surfaces.
Although it is possible to pick up respiratory viruses from contaminated surfaces, most experts say we are more likely to get sick through contact with infected people.
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