The clinical symptoms of Chicken Pox and Small Pox differ which may vary from a mild illness with only a few scattered lesions, to a severe febrile illness with widespread rash. Unapparent infection is estimated to occur in no more than 5% of susceptible children. In a majority of cases, the disease tends to be mild and typical.
During the Small Pox post–eradication era, the diagnosis of Chicken Pox is of great importance because of its resemblance to mild Small Pox. Laboratory diagnosis is rarely required as clinical signs are usually clearcut. The most rapid and sensitive means of diagnosis is examination of vesicle fluid under the electronic microscope, which shows round particles (brick–shaped in Small Pox) and may be used for cultivation of the virus.
Scrapings of the floor of vesicles show multi–nucleated giant cells colored by Giemsa stain (not in Small Pox). Serology is used mainly for epidemiological surveys