Above are schematics of 4 very different viruses (schematic from the PDB). From left to right: The giant of the 4 is the mpox or monkeypox virus, that is a current cause of concern, the WHO declared mpox a “public health emergency of international concern” last week. We appear to have a very poor understanding of how mpox is transmitted except via “close [(broken?) skin to (broken?) skin, prolonged?] contact”. Next is HIV, which is transmitted via sex and blood. The third is SARS-CoV-2, the cause of COVID-19 and the recent pandemic, which is mainly transmitted across the air. Finally, the tiny one is poliovirus, the cause of polio, which is transmitted in food/water.
Mpox viruses are 250 nm (nanometres) across, HIV and SARS-CoV-2 are around 100 nm across, while polio is only 30 nm. So the volume inside an mpox viruses is hundreds of times larger than that of poliovirus. It needs to be, mpox’s genome is 190,000 nucleotides of (double-stranded) DNA while polioviruses have only around 7500 nucleotides of (single-stranded) RNA. Mpox’s genome is 25 times the size of its fellow virus, poliovirus.
On obvious question is: What are the consequences of this diversity in viruses, for example, how does their very widely ranging size and structure affect transmission? We don’t know. There seems little correlation between something simple like size and how it is transmitted. Although the virus that – as far as I know – holds the record for longest-range transmission (tens of kilometres) is a very small one (similar in size to poliovirus).
For mpox, for example, there are examples (in animals) of airborne transmission but this seems rare in humans, for unknown reasons. This is puzzling as airborne transmission looks like such a good route, we breathe in and out every few seconds, potentially inhaling any airborne pathogen as we do so. But if a virus is transmitted via say blood, then the virus needs a cut or lesion to allow the blood out of the infected person, and another one in the skin of a susceptible person to get in.
So the air looks a much better bet for transmission than blood. In many ways the surprise is that not all viruses are airborne, airborne viruses just have to wait for the next breath, for their opportunity to leave their current host, and have a chance of infecting the next person. Presumably, viruses like HIV have evolved to exploit a different niche, that of a sexually transmitted disease, but it still leaves up in the how mpox can occasionally be transmitted across the air, but apparently mostly by what look like much less efficient pathways.
Chance & Necessity 