(b) The infection is intensified by aggregation of the sick.
These propositions are indisputably true.
2d. The poison or infection undergoes some change after leaving the human system. This appears to be susceptible of proof, because communication of the disease from person to person is not a common event. When this does apparently occur, there is often very strong reason for a belief that the contagion was resident in some fomites connected with the patient's bed or clothing.
3d. There are no sustained observations which prove that yellow-fever poison is ever created de novo.
The autochthonous birthplace of the poison is unknown. The suggestion of Niebuhr, that yellow fever may have been one of the causes of death during the plagues of Athens, can not be authoritatively denied. It may have been called into existence at the moment when all things else were created which were to perpetuate each its kind.
4th. Some of those conditions and circumstances which favor or retard the development or maturation of yellow-fever poison outside the human body are quite well understood. Warm, damp weather is most prominent among those climatic conditions which are favorable to the growth of yellow-fever epidemics.
5th. A freezing temperature ordinarily destroys the contagium of yellow fever. A high degree of artificial heat produces a similar result. It is highly probable that certain chemical agents would also effect its destruction if brought in contact with it.
6th. If yellow-fever fomites are hermetically enclosed in situations protected from cold or other agents which are destructive to their infection, its vitality may be preserved for an undetermined length of time, and its toxic qualities again made manifest when unacclimated persons are exposed to it.
7th. Yellow-fever poison possesses ponderability. This characteristic is so distinctly marked that it has been frequently termed a "low-lying poison."
8th. It is incapable of being air-borne through any great distance, at least without being deprived of its toxic effects.