There Are hundred million times more viruses on Earth than stars in the universe. Why do only some are pathogenic?

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According to a research paper published in Nature - An estimated 10 nonillion (10 to the 31st power) individual viruses exist on our planet—enough to assign one to every star in the universe 100 million times over. Viruses can infect every living thing - from plants and animals down to the smallest bacterium. For this reason, they always have the potential to be dangerous to human life. Still, only an infinitesimally small fraction of the viruses that surround us actually pose any threat to humans. This infection can happen several ways: by air (thanks to coughing and sneezing), via carrier insects like mosquitoes, or by transmission of body fluids such as saliva, blood or semen.

A virus is a set of genes, composed of either DNA or RNA, packaged in a protein- containing coat called a capsid. Some viruses also have an outer lipid bilayer membrane external to the coat called an envelope. Viruses with a lot of genetic flexibility, and particularly those that encode their genomes as RNA rather than DNA, are well-suited to crossing the species divide. Compared to viruses and cells that rely on DNA, RNA viruses tend to be sloppy when copying over their genetic code, introducing mutations at a high rate. This error-prone process creates an immense amount of diversity into populations of RNA viruses, allowing them to adapt to new environments—including new host species—at a rapid pace according to nationalgeographic

Pathogens cause illness to their hosts through a variety of ways. The most obvious means is through direct damage of tissues or cells during replication, generally through the production of toxins, which allows the pathogen to reach new tissues or exit the cells inside which it replicated.  Humans have been battling viruses since before our species had even evolved into its modern form. For some viral diseases, vaccines and antiviral drugs have allowed us to keep infections from spreading widely, and have helped sick people recover. For one disease — smallpox — we've been able to eradicate it, ridding the world of new cases.

In recent decades, several viruses have jumped from animals to humans and triggered sizable outbreaks, claiming thousands of lives. The viral strain that drove the 2014-2016 Ebola outbreak in West Africa kills up to 90% of the people it infects, making it the most lethal member of the Ebola family. But there are other viruses out there that are equally deadly, and some that are even deadlier. Some viruses, including the novel coronavirus currently driving outbreaks around the globe, have lower fatality rates, but still pose a serious threat to public health as we don't yet have the means to combat them. All human coronaviruses (HCoVs) are primarily respiratory pathogens. During the winter of 2002–2003, an alarming new disease appeared: severe acute respiratory syndrome (SARS), which was quickly attributed to a new CoV, the SARS-CoV. MERS-CoV was grown in June 2012 from a sputum sample obtained from a man in Saudi Arabia who died of overwhelming pneumonia.

Some of the viruses infecting humans are indeed capable of causing severe and often lethal diseases, but other viruses can be manipulated to be beneficial to human health. These viruses offer the potential to cure cancer, correct genetic disorders, or fight pathogenic viral infections.

Source: nationalgeographic, Livescience, howstuffworks.com, Mietzsch, Mario, and Mavis Agbandje-McKenna. "The good that viruses do." (2017): iii-v.

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