The Care and Feeding of Viruses

| May 17, 2020 | Leave a Comment

Image of Corona virus

Media Type: Article - Recent

Publisher: Millennium Alliance for Humanity and the Biosphere

Author(s): Dana Visalli

Categories: ,

Viruses are surprisingly ubiquitous, abundant, and important to the dynamics of life on Earth. They are virtually everywhere: in the air, in the water, in the soil, in all living things. There are about  one hundred billion virus particles in a liter of sea water; what are they doing there? There are an estimate 300-400 trillion virus particles in each human body, along with 40 trillion bacteria, and 30 trillion human cells. In terms of the numbers, it could be argued that we are each at least as much a community of virus particles as we are individuals.

It turns out that most viruses are bacteriophages—they prey on bacteria. That is why there are so many in the oceans, and in all living things. Most of that multitude of bacteria inside of us and on our skin are either beneficial to us or at least not harmful, and the same is true of the even greater number of virus particles. But the viruses are critical to restraining the growth of bacteria in all organisms and in all ecosystems.

What is a virus? It is packet of genetic material, DNA or RNA, surrounded by a protective shell. That’s it. A virus particle has no metabolism, no senses, no organs, no locomotion, no brain; it is just genetic material. But, the genetic material is coded in such a way that once it is inserted into the cell of a host (which could be from any of the six kingdoms of life, archaea, bacteria, protists, animals, plants or fungi), it is able to take over control of the activity of the cell to produce more of itself; more virus particles. The organelles in the cell, the ribosomes, the cytoplasm, all become slaves to the virus. The attraction between the host cell membrane and the virus shell is simply electro-chemical; the virus is attracted to the cell membrane like a magnet to metal.

Are viruses alive? The answer depends on one’s definition of life, one could say ‘yes and no.’ We like clear-cut, black or white distinctions in our understanding of life, and viruses reside in a more nuanced realm. They are definitely organic structures, formed by the chemistry of life, and they are evolved structures, products of natural selection and community relationships. Viruses exist in tight co-evolutionary relations with their host specie or species, just as bees live in a tight co-evolutionary relationship with flowers. So they are intimately bound to and a part of the dynamics of life.

Virus particles are small, very small. The smallest are 20 nanometers (nm) in diameter. A nanometer is one billionth of a meter, so 20 nm is 20 billionths of a meter. That’s only about 100 times larger than an  atom. They are invisible in light microscopes, and were unseen until the development of the electron microscope in the 1930s. You could line millions of them up on the head of pin. Put one grain of salt on the table; you could line up about ten skin cells along one side of it. You could line up about a hundred bacteria. Compared to viruses, however, bacteria are giants. You could line up ten thousand viruses alongside that same grain of salt.

Virus particles are small, but what they lack in size they make up for in numbers. Globally, there are an estimated 1e31 virus-like particles. That is a 1 followed by 31 zeros (10 nonillion, in case you wondered). That is more than the estimated number of stars in the universe (1e24), or grains of sand on the Earth (1e22, these are  estimates, the counting is not yet completed).

What are all these virus particles up to? Mostly they are consuming bacteria, taking over the genetic machinery of individual bacterium to produce multiple copies of themselves, and then ‘lysing’ the bacterial cell (breaking it open) to spill both the organic matter of the bacterium and the newly hatched viral offspring into the environment.

Much of our knowledge about the roles of viruses in natural environments comes from studies of marine microbial communities. In the world’s oceans, about half of the organic matter produced  by photosynthestic organisms (these producers are largely composed of diatoms and dinoflagellates, both considered forms of algae) supports bacteria that then feed on this energy source. To repeat that: half of the biological productivity of the ocean feeds bacteria. not fish or whales. The innumerable viruses in the oceans then feed on the bacteria. Thus viruses are an integral component of the ecology of the oceans. The bacterial cells lysed by the viruses  become dissolved organic matter which can be used by other heterotrophic (‘other feeding,’ not photosynthetic but feeding the products of photosynthesis, just as you and I do) bacteria.

Microbes are the planet’s great geoengineers. Algae and photosynthetic bacteria churn out about half of the oxygen we breathe. Algae also release a gas called dimethyl sulfide that rises into the air and seeds clouds. The clouds reflect incoming sunlight back out into space, cooling the planet. Viruses kill these geoengineers by the trillions every day. As the microbial victims die, they spill open and release a billion tons of carbon a day. Some of the liberated carbon acts as a fertilizer, stimulating the growth of other microbes, but some of it sinks to the bottom of the ocean, where it can be removed from the biosphere for millions of years. What controls this massive geoengineering project? The activity of viruses.

Most viruses are either critically important to human well-being (through ecological engineering) or have no impact on human biology. But viruses are opportunistic and genetically adaptable to new food resources. The human population has experienced an explosive increase in the past century. It took 200,000 years for humans to reach their first billion (in 1800), 130 years to reach the second billion (1930), 30 years to reach the third billion (1960)…..and the human population is now increasing by one billion every fifteen years.

Viruses do not single out Homo sapiens for a feeding frenzy, it’s just that we are so abundantly available. Humans and their livestock now account for 96% of all mammals on the planet; we have effectively taken over the world. ‘If you look at the world from the point of view of a hungry virus,’ the historian William H. McNeill has noted, ‘or even a bacterium—we offer a magnificent feeding ground with all our billions of human bodies, where, in the very recent past, there were only half as many people. In some 25 or 27 years, we have doubled in number. A marvelous target for any organism that can adapt itself to invading us.’

There is a limitless reservoir of viruses in the natural environment. The current corona virus outbreak (one name for it is SARS-Covid 2) is just one of a number of ‘zoonotic’ (from the Greek, ‘animal disease’) viruses that have transferred from this limitless reservoir of other lifeforms to humans in recent history; some others include Ebola, SARS-Covid1, HINI ‘Swine Flu, and HIV (AIDS). It is worth remembering that viruses have no locomotion of their own; they can only spread through some form of external transport. Both the transmission of viruses to humans from other species and the transmission from one person to another is contact and density dependent.

The World Health Organization estimates that each year the flu infects 5 to 10 percent of all adults and 20 to 30 percent of all children. Somewhere between a quarter and half a million people die of the flu each year. It is thought that well over 50 million people died of the Spanish flu in 1918-19. Smallpox is density-dependent viral disease that requires close contact with an infected person for transmission; it has killed between 2 and 3 billion people in the 3000 years it is known to have existed. As mentioned about, there are up to 400 trillion virus particles in every human, and somewhere in the neighborhood of 31,000,000,000,000,000,000,000,000,000,000 virus particles extant on the planet. Wearing a mask can keep them inside your body, but it cannot keep them out. We live on a planet of viruses.

The larger the human population grows and the greater the density of human communities, the more virus particles will be passed through the population, and the greater the potential for viral diseases. From this ecological perspective the increasing density of the human population of the planet is akin to a death wish, initiated by the biological urge to reproduce, but carried out in ignorance of the ecological basis of communities of life. It is estimated that in the oceans roughly 1e25 (25 zeros) microbes, or about 100 million metric tons, die every 60 seconds  due to viruses. Dense accumulations of humans are also ultimately under viral control.



Quammen, David (2012) Spillover: animal infections & the next human pandemic. WW Norton & Co.

Zimmer, Carl (2015)  A planet of viruses. University of Chicago Press.

Rohwer, Forest, Prangishvili, D., Lindell, D., (2009) Role of viruses in the environment Environmental Microbiology Available at:

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