Measure the number of viruses on the planet
If you get the flu, about 10,000 new flu viruses are produced for every infected cell in your respiratory tract. In a few days, the number of flu viruses produced in your body will be as high as 100 trillion. That's more than 10,000 times more than all humans on the planet combined. An average healthy person has about 3,1012 viruses in his body, and if you put all the viruses in all the people in the world together, you can fill about 10 ** buckets (1 bucket is about 159 liters).
But there are more viruses in the soil, with some areas of the wetlands of Delaware in the United States containing about 4 billion viruses per milliliter of soil. Most of the viruses in the soil are viruses that live in bacteria, and this virus is called bacteriophage.
If one person has so many viruses, how many viruses are there on our planet in total?Scientists can't count every virus on the planet, what they do is they take samples of different environments — soil, freshwater, oceans, etc. — and then make reasonable inferences. However, as the investigation deepens, the results of the inference will change.
In the past, scientists used an electron microscope to count the viruses in a sample one by one to investigate the number of viruses. But this is a terrible solution, in part because some of the virus's hosts — bacteria and protozoa — are mostly unable to survive in artificial environments. Without a living host, the virus cannot survive, and it is impossible to know how many of them there are.
But scientists have come up with a better solution: instead of looking for mature viruses, they can simply measure the amount of fragments of their DNA in a particular sample. Through this method, scientists have discovered amazing facts about the ocean. The ocean was once thought of as a desert for viruses, but in fact it is a broth of viral DNA where most of the world's viruses gather. As with terrestrial soils, scientists have found that viruses in the ocean are basically bacteriophages.
By detecting viral DNA, scientists deduce that the total number of viruses on Earth is about 1,031. This number is about 10 million times the total number of stars in the entire universe. If you connect each virus one by one on Earth, the resulting chain of viruses will span across the Moon, across the Sun, across Proxima Centauri, across the edge of the Milky Way, across the Andromeda Galaxy, all the way up to 200 million light-years away.
Don't forget about RNA viruses
However, scientists have known for a long time that there is another different form of virus that uses different molecules as genes.
DNA is a double helix structure with genes on both strands. When genes in our body translate into proteins, DNA is copied into a single-stranded molecule called RNA. In some viruses, RNA replaces DNA and takes on the task of carrying genes. The influenza virus is a typical example of many RNA viruses. Unlike bacteriophages, RNA viruses never infect bacteria. Instead, they infect us as well as other animals, plants, fungi, as well as protozoa, all life known as eukaryotes.
Scientists didn't consider RNA viruses when they considered the total number of viruses, so 1031 may not be right. Recently, scientists from the University of Hawaii in the United States estimated the number of RNA viruses for the first time, scooping up about 115 liters of seawater from the dock on Oahu (the main island of the Hawaiian archipelago) and then extracting the RNA of all the viruses in the sea through a series of filtration and other steps. Scientists then measured the overall mass of the RNA, and based on the average mass of RNA contained in a virus, they could estimate how much RNA virus was inside.
The results are striking: about half of all viruses in the ocean are RNA viruses. This is strange because RNA viruses can only infect eukaryotes, and the number of bacteria (phages as hosts) in the ocean far exceeds eukaryotes. However, this is not difficult to understand considering that a eukaryotic cell produces far more viruses than a bacterium.
If this study is correct, then the total number of viruses on Earth, probably 1031 plus 1031, would make your virus chain 400 million light-years long. However, there is only one sample size at the moment, and scientists need to take more samples from around the globe to get more accurate results.