Viruses are not alive by most definitions. They are generally smaller than most bacteria, about a tenth of the size of a human blood cell. The structure of a virus is very simple: Each one consists of either DNA or RNA, enclosed in a protein pocket called a capsid. Viruses can't generate their own energy nor reproduce without the assistance of a host.

Virus

Viruses cannot move or reproduce without the help of a host cell. When a virus finds a host, it can increase and spread quickly.
To find a host, viruses have receptors on their surfaces that match up with their ideal target cell. It then takes over the cellular machinery inside the host's cell to reproduce by multiplying the virus's genetic material and proteins.

Viruses multiply

<ul>
<li>Inside their cellular host, a virus can create a huge number of copies and spread the infection to other cells.</li>
<li>Viruses may spread from person to person through the mist of droplets from your mouth every time you cough or sneeze. </li>
<li>Some viruses spread through contact with other bodily fluids. Ebola virus spreads from contact with infected blood, feces, or vomit, and not through the air.</li>
<li>Other viruses travel through an intermediary, like a mosquito, which then infects people by biting them.</li>
</ul>

How viruses spread

<div>
Viruses are curious things. The tiny tagalongs aren't exactly alive by most definitions, but they're not really inanimate either. They've flourished and diversified for billions of years and perhaps even had a hand-or a squishy protein coating-in helping the first complex cellular life come to be.
While these microbes have a dizzying array of functions and health effects, the <a href="https://www.britannica.com/science/virus">structure of a virus </a>is surprisingly simple. Each one consists of genetic material-either DNA or RNA-encapsulated in a protein pocket called a capsid. Some are additionally enveloped in a soft, lipid wrapping. These tiny virus packages are just <a href="https://learn.genetics.utah.edu/content/cells/scale/">tens to a few hundreds</a> of nanometers across. This makes them smaller than most bacteria, which can be a small as roughly a tenth the size of a <a href="https://learn.genetics.utah.edu/content/cells/scale/">human blood cell</a>. Such a tiny size means that you can't even spot most viruses with a light microscope.
The one exception, a group known as <a href="http://www.virology.ws/2013/08/01/pandoravirus-bigger-and-unlike-anything-seen-before/">giant viruses</a>, has members with astonishingly large genomes. These mega-viruses are hundreds of times larger than most, with capsids that span roughly <a href="https://www.sciencedirect.com/topics/immunology-and-microbiology/giant-virus">400 to 500 nanometers</a> across and full viral forms that can measure up to <a href="http://www.virology.ws/2011/12/14/megavirus-the-biggest-known-virus/">750 nanometers</a> across.
How do viruses multiply? 
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 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4788752/">genetic material inside</a> and <a href="https://ucmp.berkeley.edu/alllife/virus.html">hijack its host's cellular machinery</a> 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 <a href="https://mbio.asm.org/content/4/5/e00598-13">one estimate</a>, at least 320,000 different viruses can infect mammals alone, and even this massive number <a href="http://www.virology.ws/2013/09/06/how-many-viruses-on-earth/">may be on the low side</a>. This viral army can cause symptoms as mild as a cough or as deadly as <a href="https://www.cdc.gov/vhf/ebola/symptoms/index.html">internal bleeding</a>. Some viruses may even cause the runaway cellular growth that is the root of cancer, as is thought to be the case with <a href="https://www.cancer.org/cancer/cancer-causes/infectious-agents/infections-that-can-lead-to-cancer/viruses.html">human papillomavirus and cervical cancer</a>.
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 <a href="https://www.nationalgeographic.com/science/phenomena/2013/02/20/an-infinity-of-viruses/">body will be riddled with some hundred trillion viruses in just a few days</a> -more than 10,000 times the number of people on Earth.
How viruses spread from person to person depends on the type. Many hitch a ride in the <a href="https://www.webmd.com/cold-and-flu/qa/how-do-viruses-spread-from-person-to-person">mist of droplets</a> 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 <a href="http://sitn.hms.harvard.edu/flash/2014/the-reason-for-the-season-why-flu-strikes-in-winter/">cool, dry environments</a>, 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, <a href="https://www.who.int/mediacentre/news/ebola/06-october-2014/en/">Ebola virus spreads</a> 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 <a href="https://www.cdc.gov/niosh/topics/outdoor/mosquito-borne/default.html">mosquito-born diseases</a> 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 <a href="https://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue">World Health Organization</a>. Other notorious mosquito-born diseases include Zika, Chikungunya, and West Nile.
Where did the first viruses come from? 
Some scientists believe that viruses were fairly late to the evolutionary game, forming as remnants from cells that had somehow lost the ability to replicate. But other experts suggest that viruses could predate Earth's most ancient critters.
The giant viruses have a surprising amount of independence compared to their tiny counterparts, so <a href="https://news.nationalgeographic.com/news/2014/07/140716-giant-viruses-science-life-evolution-origins/">could have provided the building blocks of the diversity of life we know today</a>. By <a href="https://www.ncbi.nlm.nih.gov/pubmed/11523012">one hypothesis</a>, the first complex life originated from a cell enveloping a virus or, alternatively, a <a href="https://www.youtube.com/watch?v=7KXHwhTghWI">failed viral takeover</a>. Either way, the virus became a permanent cellular resident, forming the first nucleus.
However, scientists can't even agree on whether viruses are truly alive. To be <a href="https://www.nature.com/scitable/topicpage/the-origins-of-viruses-14398218">considered a living thing,</a> an organism must be able to grow, reproduce, and generate energy on its own. Some researchers also suggest that living things must be able to respond to stimuli and evolve over time. Viruses can't generate their own energy, and though they can reproduce and even evolve with the assistance of a host, those functions are impossible for one of the tiny entities out on its own.
Instead, Albert Erives of the University of Iowa <a href="https://now.uiowa.edu/2018/02/giant-viruses-may-play-intriguing-role-evolution-life-earth">suggests that viruses </a>are more like vines wrapping around the many branches of the tree of life. They can access and infect critters on each branch, racing to the top to evolve as their hapless hosts morph over time.
</div>

Viruses are curious things. The tiny tagalongs aren't exactly alive by most definitions, but they're not really inanimate either. They've flourished and diversified for billions of years and perhaps even had a hand-or a squishy protein coating-in helping the first complex cellular life come to be.

Viruses, explained

Virus

Viruses multiply

How viruses spread

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