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A virus is a microscopic, infectious cellular invader. Viruses insert themselves into living cells where they replicate. They can infect most life forms: from bacteria to plants to animals. Every cellular organism studied so far has its own viruses. Millions of viruses are found across all ecosystems and life forms on Earth, with about .

A bacteriophage—or phage for short—is a virus that specializes in infecting bacteria. Most viruses are bacteriophages. These particular types of viruses are made of proteins that infect the bacterial cell, then they enclose the DNA or RNA genome within the cell.  and are found everywhere bacteria exist, which is across all environments on Earth.

Viruses specialize on all different forms of cellular life; each virus evolved to infect different forms. Animal viruses infect only animals, and two fields of study separate their study. For non-human animals, the field is known as "veterinary virology," while "medical virology" is the study of viruses and human beings. Viruses that affect humans are the most studied with .

°Õ³ó±ðÌý that helps it enter its target cell is called a capsid. It protects the gene material of the virus. Structures of capsids vary widely and may consist of numerous proteins.

Some capsids build what are called "" from the cell itself. These are lipid membranes the virus builds around itself, with lipid material of the cell's inner membrane. Viral envelopes are thought to help the virus infect the target cell. Lipids are the cell's fatty acids and are not water soluble.

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Endocytosis is the term for when . Viruses infect their cells in various ways. In some cases, the virus enters the cell but leaves the capsid behind, on the outside of the cell. In enveloped viruses, the viral envelope fuses directly with the cell membrane then it enters the cell. Inside the cell, the capsid degrades and the genetic material of the virus is released.

Some viruses have a special trait that allows them to enter and infect a cell, then go dormant. The virus may replicate in the cell at first, then stop.  refers to the time that viral genetic material can remain in the cell before being reactivated. If reactivated, the virus can reinfect the host without the host being re-exposed. HIV is known to have viral latency.

is when an infectious disease is transmitted from other vertebrate animals to humans. These kinds of infections can occur in natural conditions because vertebrate animals are genetically similar to humans. Some examples include the black plague, transmitted by rats; rabies, transmitted by bats, raccoons, and dogs; and anthrax, transmitted by sheep. More recently emergent human diseases like HIV, Ebola, and SARS likely arose from zoonosis.

Infections can spread from one person to another through . This happens with handshakes, kissing, other skin-to-skin contact, and sexual intimacy.  a person realizes they are spreading a disease.

refers to transmission when coughs, sneezes, and breath travel some distance from an infected person to a non-infected person. Generally people have thought that close proximity is necessary for droplet spread, but  (and motivated by the coronavirus pandemic), shows that infectious droplets can travel as far as 23 to 27 feet.

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Infectious particles like viruses can travel on dust or simply be suspended in the air. These particles can settle onto various surfaces, then can stir up and re-suspend in the air. If an uninfected person is exposed to these infectious particles, that person can become infected via . Some people have caught measles by entering a room where people with measles had recently been.

occurs when an infectious disease arises in a community in which there is no connection to a known case, and/or no known history of travel into or out of a region with the disease.

The "" is also known as R0 ("R-nought"), a measure that describes how easily a virus spreads. Specifically, R0 is an estimate of how many other people get infected by one infected person. For example, seasonal flu has an R0 of about 1.3 while 1.5 or more people can be infected by each carrier, with some research indicating an R0 factor being as high as 6.68. Changes to R0 can happen with how often people see others, location, and strength of the efforts to lower spread. 

´¡²ÔÌý is defined by Merriam Webster's dictionary as "an outbreak of disease that spreads quickly and affects many individuals at the same time." An epidemic is a situation in which a .

´¡Ìý results from an epidemic that has grown past geographic boundaries. It is a type of epidemic. It occurs over a wide geographic area and impacts "an exceptionally high proportion of the population"—likely a whole country or the entire world. On March 11, 2020, the World Health Organization categorized the.

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are medications that are used to inhibit viruses and their development. Antibiotics typically destroy the infectious agent, but since viruses are not exactly alive, antiviral drugs are designed to interfere in some way with the virus. 

Vaccines prevent disease. ´¡Ìý, but it is rendered harmless: Either it's killed or weakened to the point it does not cause illness. When the vaccine is injected into the body, the immune system responds by making antibodies, leading to the same immunity a person would have if they'd become sick and recovered. When  whole populations because of the diminished chances of an outbreak. 

°Õ³ó±ðÌý is a viral infection of the nose and throat that causes various symptoms like sore throat, runny nose, headaches, cough, and low fever. Many viruses can lead to the common cold, and most healthy people recover from a cold in six to 10 days.

The term  defines a "family of viruses that cause illness ranging from the common cold to more severe diseases, such as Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS-CoV). The novel coronavirus [most] recently discovered has been named SARS-CoV-2 and it causes COVID-19." Coronaviruses are not the flu.

The novel coronavirus that causes the disease COVID-19 is known as . Although this new disease may have certain similar symptoms to seasonal flu, It is from a completely different family of virus, and its particular set of traits make it highly contagious and far deadlier. The has an information sheet to help people decide if they might have the new coronavirus, and what to do if they are sick.

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separates those who have a disease from those who don't, or who are not known to be sick. The public has been  during the pandemic if they or their family members have symptoms.  also provides guidance on home isolation and what to do once you no longer have symptoms.

is when people stay away from each other, avoid all crowds, and cancel large and small events and gatherings. Social distancing effectively keeps this virus from spreading between people, and thus saves lives—particularly in the current pandemic because of its extreme contagion and high fatality rate. Distancing has included closing schools, working remotely, staying at least 6 feet apart from other people, and connecting with loved ones using online platforms, phones, and social media.

If people have been exposed to someone with the contagious disease,  separates and restricts their movement to see if they get sick. Quarantine helps ensure that if someone is already exposed, that they stay away from others. People can  without knowing they're contagious.

"" is a key way to save many, many lives by . This allows time for health-care workers, hospitals, and related systems to help infected people, without becoming overwhelmed by exponentially rising numbers of seriously ill patients. It is very effective, may last from weeks to months, and could save tens of thousands of lives.

happens to a population that has been exposed to an infectious agent and, as a result, becomes immune. If this happened with the novel coronavirus—but without flattening the curve—the loss of life would be catastrophic because the virus is so contagious and deadly.

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