A constant period of incubation is not to be expected. In most
instances, as will be seen from the following table, the difference
between the maximum and the minimum period is not very great. It seems
remarkable, however, that a disease should show such extremes as
typhoid fever:
Normal. Maximum. Minimum.
Variola 12 days. 14 days. 9 days.
Varicella 14 “ 19 “ 13 “
Measles 10 “ 14 “ 4 “
Rubella 18 “ 21 “ 8 “
Scarlatina 2 “ 7 “ 1 day.
Influenza 3 “ 5 “ 1 “
Diphtheria 2 “ 7 “ 2 days.
Typhoid fever 12 “ 23 “ 5 “
Mumps 19 “ 25 “ 12 “
It is a peculiar fact that the diseases in which the period of
incubation is shortest are those in which the infection persists the
longest.
The period of quarantine must be guided largely by the period of
incubation, hence the subject is an important one for a variety of
reasons. The “Medical Magazine” (London) states that the period of
quarantine should be at least a day longer than the maximum period
of incubation for each disease. This is a very uncertain rule,
however, for the patient should be free from all signs of illness,
and especially from fever. The period of infection is very doubtful.
It may be greatly prolonged by complications. This is especially true
of small-pox, diphtheria, typhoid and scarlet fevers. The period
during which a disease may be infectious cannot be stated definitely.
It varies with different diseases, and must be determined according
to the symptoms and character of the case. Measles, chicken-pox, and
mumps lose the direct power of infection very early, and the infective
principle does not remain active for a long period in the room in
which the patient has been ill. Measles, mumps, and chicken-pox may be
infectious in the earlier stages before becoming definite in character.
Smallpox is not actually dangerous until the eruption appears.