Approximately 10³¹ viral particles reside on Earth, and thus there are more of them in the observable Universe than there are stars. Most such particles are phages that are incapable of actively infecting humans and can only target bacteria. They are still however to be found in your cerebrospinal fluid, amongst other strange locations.

The total space occupied of all human coronavirus particles in existence does not exceed the volume of a single coke can. If someone were to drink such a beverage, it would taste like synovial fluid. Most virions would lose viability when exposed to low pH conditions in the stomach, and so the dose would not be lethal.

According to an interesting hypothesis, most of the dissolved iron present in the World’s oceans exists bound to marine viruses. The phages accumulate Fe atoms on the tips of their tail fibres. During infection, these fibres act like a needle to pierce the membrane of the host bacterium. Being of viral origin, the bacterial type VI secretion system operates in a similar way.

The double-stranded DNA genome of bacteriophage T4 that infects the common bacterium Escherichia coli is packaged in the head so tightly, it exerts a pressure of over 20 atmospheres. That is about 5x the pressure inside an average coke can. During infection, this high pressure helps the DNA leave the virus’ head and enter the host cytoplasm.

Viruses are the simplest organisms on Earth but are also the most diverse and vary wildly in size and shape. The smallest viruses are in the Circovirus genus and measure ~17 nm in diameter. Many giant amoeba-infecting viruses from the Pandoravirus genus are over 100x larger, bigger than some bacteria, and as such can be viewed through a light microscope. Pithovirus sibericum has the grandest virion of all, measuring 1.5 μm in length.

Viral genomes are extremely compact and optimised. The tiny phage ΦX174 has a +ssDNA genome 5,386 bases long, 95% of which belong to highly overlapping protein-coding genes. There is in fact a section coding for three genes at once, using all 3 available reading frames! For comparison, the human genome has 3.2 billion base pairs, and only 1.5% code for proteins.

About 8% of our genome is made up of mysterious sequences known as ERVs. They are in fact remnants of ancient retroviruses that have managed to quietly insert themselves into our ancestors’ germline and are now stuck. The human protein Syncytin-1 responsible for placental formation is in fact a repurposed fusion peptide we have ‘borrowed’ from a retrovirus.