Smallpox vs. Chickenpox
Smallpox, like other poxviruses, has lesions that can cover the entire body. Physicians will not likely begin to diagnose smallpox until the lesions become obvious. At that point, healthcare providers will attempt to gain a history of the illness prior to the appearance of the lesions.
To determine the difference between smallpox and chickenpox, the healthcare provider will be looking at the formation of the lesions as the most important sign.
Smallpox: Lesions are hard and well-defined. All lesions will develop at the same rate and will be similar in their formation and firmness. Sometimes, the lesions will have a small indentation on their crown, known as an umbilical formation. Smallpox lesions are often preceded up to one to four days by a fever as high as 105 degrees. The lesions will be distributed on the arms and face and appear on palms and soles of the feet. Chickenpox: Lesions are not as well-defined and will be at different stages of development. They are not firm and will be easy to remove. There will not likely be any fever preceding the onset of lesions. The lesions are likely to appear first on the torso rather than the arms and face. They will rarely appear on the palms or soles of the feet.
If you or someone in your family develops lesions that appear to be chickenpox or smallpox, see a healthcare provider.
Differential Diagnoses
Other poxviruses can mimic the way smallpox looks but are significantly less lethal than smallpox. Some of these are closely related to the variola virus.
Orthopoxviruses
There are several zoonotic (infects animals and humans) versions of orthopoxvirus, the family of virus that includes variola, which is the cause of smallpox. These often look like smallpox and can be similar. Some can be serious.
Cowpox affects both cows and humans. Before clinical vaccination (which comes from the Latin word for cow) was widespread, farmers would develop some inoculation of variola through exposure to cowpox. Vaccinia is another virus that affects cattle and is the base virus for the smallpox vaccine. Mpox (formerly known as monkeypox) is the most closely related to smallpox and still naturally infects humans in some African nations. It has a mortality rate of 1 to 10 percent. Camelpox affects camels and can cross over to humans. Buffalopox is closely related to vaccinia and is common in India.
Varicella and Herpes-Zoster
Chickenpox is primarily a child illness from the varicella-zoster virus. As mentioned above, chickenpox lesions are less robust than those of smallpox and are very unlikely to appear on the palms or soles of the feet.
Shingles (herpes-zoster) is a secondary infection from the same varicella virus and appears mostly in older adults. Shingles lesions follow major nerve pathways and are nearly always on one side of the body (unilateral).
