Following up from the crAssphage press and comments Dan asked me the following question:

It was interesting to hear that there are 10 times as many viruses as bacteria in the body. If you have time to answer a question, I’ve always wondered about the relative biomass of bacteria compared to human cells, and now the relative biomass of viruses compared to human cells.

Inspired by XKCD’s what-if we can use some Fermi estimation to answer this. A typical virus is about 10^-19 kg. (e.g. Adenovirus which is about 50kb is 2.5 x 10^-19 kg ^[1]). A typical bacterium, like E. coli is about 10^-15 kg, and a typical human cell is about 10^-12 kg.

Scientists like to say that we have ~10x more bacteria than human cells and ~10x more viruses than bacteria. In the human body there are about 37 trillion cells [2] (37 x 10¹², but since we are estimating we’ll round that to 10¹⁴) . Based on these estimates we have the average human weighs about 100 kg (10¹⁴ cells x 10^-12 kg) in human cells, 1 kg in bacteria (10¹⁵ cells x 10^-15 kg), and 0.001 kg in viruses (10¹⁶ viruses x 10^-19 kg)