This patient first developed a rash the day before this picture was taken. What is the diagnosis? Smallpox or chickenpox? After discussion: At this time, it is difficult to make a definite diagnosis simply from observation of the rash. What other information might provide important clues to diagnosis?
After discussion: (1) If the patient had experienced 2 to 4 days of fever before onset of rash, this would suggest that she has smallpox. If fever developed at the time of rash, the diagnosis of chickenpox would be more likely. (2) Information as to whether the patient had experienced either smallpox or chickenpox in the past would be helpful since second attacks of either disease are very rare. (3) If mass vaccination of populations has been resumed and the patient has a vaccination scar, the diagnosis of chickenpox is more likely as few cases of smallpox are observed in previously vaccinated persons, especially those vaccinated within the preceding 5 to 10 days. (4) A history of contact during the preceding 2 to 3 weeks with either a case of chickenpox or smallpox would provide valuable information. Note, however, that even with these clues to diagnosis, one may not be able to decide definitely one way or the other. The patient should be isolated and kept under observation. If the disease progresses as described for smallpox, samples should be collected for laboratory diagnosis.

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