┌──────────────────┬─────────────────┬────────────────────────┐ │ DISEASE. │ BEGINS USUALLY │ BUT MAY POSSIBLY BE AT │ │ │ ON THE │ ANY PERIOD BETWEEN │ ├──────────────────┼─────────────────┼────────────────────────┤ │_Scarlet fever_ │ 4th day. │ 1 and 7 days. │ │ │ │ │ │_Diphtheria_ │ 2nd „ │ 2 „ 5 „ │ │ │ │ │ │_Small-pox_ │ 12th „ │ 1 „ 14 „ │ │ │ │ │ │_Chicken pox_ │ 14th „ │ 10 „ 18 „ │ │ │ │ │ │_Typhus fever_ │ 12th „ │ 1 „ 21 „ │ │ │ │ │ │_Enteric fever_ │ 14th-21st „ │ 1 „ 28 „ │ │ │ │ │ │_Cholera_ │ 1st-3rd „ │A few hours and 10 days.│ │ │ │ │ │_Measles_ │ 12th-14th „ │ 10 and 14 days. │ │ │ │ │ │_Rötheln_ │ │ │ │(_German measles_)│ 14th „ │ 12 „ 18 „ │ │ │ │ │ │_Mumps_ │ 19th „ │ 16 „ 24 „ │ │ │ │ │ │_Whooping cough_ │ 14th „ │ 7 „ 14 „ │ │ │ │ │ │_Influenza_ │ 2nd „ │ 2 „ 6 „ │ └──────────────────┴─────────────────┴────────────────────────┘ The period of incubation is several weeks in hydrophobia and syphilis, and may be several years in leprosy. Following the period of incubation, come the premonitory symptoms, which usually are somewhat sudden in onset. For the chief =symptoms of onset= see page 318. Persons vary in susceptibility to attack by different infective diseases. The intensity of an attack depends on the condition of the patient, and on the number and the virulence of the particular microbes infecting the patient. In certain families attacks of particular diseases are more severe, and attacks are more liable to occur than in others.