Chickenpox

Chickenpox, or varicella, is a contagious disease caused by the varicella-zoster virus. It is usually the primary infection, meaning the virus's first encounter with a body that is not yet immune. Herpes zoster, or shingles, is something else: it is the same virus, which has stayed latent, or dormant, and then reactivates.

The virus enters through the respiratory mucosa, the moist layer lining the inside of the airways. From there it reaches nearby lymph nodes and replicates, which means it makes copies of itself. Then it passes into the blood, a stage called viremia, and in a second viremia it reaches the skin. That is where the lesions appear.

On the skin you can see the whole progression. First come red spots, called macules, and papules, small bumps that are also red, and they all itch. Very quickly they turn into vesicles, tiny blisters of clear fluid surrounded by a red halo. Each vesicle looks like a drop of dew on a rose petal. The older ones become pustules and then crusts, so new and old lesions coexist on the same skin, and that is characteristic of chickenpox.