It is ordinarily prepared by the fermentation of sugar or starch, brought about by the addition of putrefying cheese, calcium carbonate being added to neutralize the acids formed in the process.
When these processes continue for a long time in deep water shut off from free circulation so that it does not become aerated by contact with the atmosphere the water becomes unfit to support the life of fishes, and when the accumulation of putrefying organic matter gives rise to sulphuretted hydrogen as in the Black Sea below 125 fathoms, life, other than bacterial, is impossible.
This great assemblage is always a dangerous centre of infection, and the days of Mina especially, spent under circumstances originally adapted only for a Bedouin fair, with no provisions for proper cleanliness, and with the air full of the smell of putrefying offal and flesh drying in the sun, produce much sickness.
Houseflies can transmit diseases when feeding on liquefiable solid food, which may be moist, putrefying material or food stored for human consumption.
Thus there is always a quantity of putrefying matter contaminating the air.