"so insignificant a creature as the comma bacillus"
The comma bacillus, which causes cholera, was discovered in 1849. Proust's father Adrien, a doctor, worked at combating the disease and is known for his application of the cordon sanitaire
as a means of preventing its spread. The comma bacillus is thus named as a result of its curved shape, which is similar to a comma without the 'head'.