By the seventeenth century, the observation that immunity from severe smallpox followed a mild episode of the disease had led to the practice of ‘variolation’ in China and the Ottoman Empire. Variolation was so named because material was taken from dried scabs or pustules (the Latin varus, 'marks on the skin’) and used deliberately to infect healthy recipients. Infected material could be applied to an area of scarified skin or could be introduced into the nose as a dust. The material was taken from people with a milder form of the disease (possibly caused by variola minor virus). Although there was no understanding of either the immune system or infectious agents at that time, the effect was to expose the recipient to a supposedly ‘weaker’ strain of the virus, which elicited a protective immune response against subsequent infection with a more virulent strain.
What key features of the immune response does the practice of variolation demonstrate?
It shows the specificity of the immune response (variolation with smallpox material protected recipients specifically against this disease) and the existence of immunological memory (it resulted in long-lasting protection).
Variolation was usually followed by a fever and the treated area of skin developed an acute inflammatory response with the characteristic ‘pox’ lesions. Although recovery usually ensued, 2–3 per cent of people could be killed by the smallpox infection that resulted from the procedure.
Why would a ‘medical’ procedure with a 2–3 per cent fatality rate be tolerated?
The risk of fatality from variolation was still much less than that of contracting a fatal smallpox infection.
Variolation was introduced into Europe early in the eighteenth century (you will learn more about this period when you read the article at the end of this section), where it gradually became known as inoculation (from the Latin for ‘to graft or implant’).