How can tiny insects like fleas, ticks, mosquitos, etc. physically carry enough of a virus to impact a full grown adult human.
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Viruses are tiny. For example, Sars-CoV-2 which causes COVID-19 is around 80-120 nanometers in diameter. A nanometer is a billion of meter, you need around 1000 of them side by side for the width of a human hair.
If you do the calculation of the total volume of all Sars-CoV-2 om the world at one moment in time during the pandemic the total volume is less then you expect. A human would have around 10 billion viruses in them at the peek of an infection and if you assume 3 million get infected each day the total volume of all viruses is less than a soft bottle
A cube with a side the with of a human hair can fit 1 billion viruses if you assume they are cubes with the diameter as sides, you could fit even more spheres. So you could fit all Sars-CoV-2 viruses a human has in their body during the peek of a infection in a mosquitoes Theoretical 1 virus is enough for an infection, in practice you likely need to stop the immune system, from beating them initially
Sars-CoV-2 is not transported by insects but the link to the calculation gives you an idea of how tiny viruses really are.
Viruses and bacteria replicate fast and furiously. So that tiny amount that is injected quickly grows to be enough to make a human sick if their immune system isn’t able to kill it quickly enough.