From 1908 to 1929, Clemens von Pirquet was one of the world's most acclaimed pediatricians. Von Pirquet (1874-1929) trained at Vienna's Universitats Kinderklinic under Theodor Escherich, the first Pediatric Infectious Diseases physician , and became the first Professor and Chair of Pediatrics at Johns Hopkins in 1909. He then succeeded his mentor Escherich as Professor and Chair of Pediatrics in Vienna, the most prestigious European pediatric position, when Escherich died unexpectedly in 1911. He held that position in Vienna until his shocking double suicide at age 54 with his wife in 1929. Von Pirquet's pioneering contributions from 1903 to 1910 related to host reactions to foreign substances, providing much of the foundation for modern "Immunology". In 1905, he and his student Bela Schick described and named "serum sickness" in children administered animal antiserum. He recognized that animal antiserum resulted in both protection against an infection but also sensitization (sometimes with serious or fatal consequences), ie, that immune responses caused some diseases. In 1906, he proposed the term "allergy" for the altered reactivity induced by what he termed an "allergen", a foreign substance. He recognized that sensitization to an allergen leads to accelerated responses on subsequent allergen administration, analogous to differences between primary and subsequent smallpox vaccine responses. In 1908, von Pirquet presented his invention, the "tuberculin skin test", recognizing its ability to identify individuals with previous tuberculosis infection, then the most prevalent infectious disease. This led to the new understanding that many or most tuberculosis-infected individuals are asymptomatic but at risk for future active disease, introducing the concept of "latent tuberculosis". Von Pirquet was a consummate pediatrician-scientist, translating scientific discoveries directly into improved care of children, and he also pioneered study of the social, nutritional, and public health aspects of pediatrics, especially during and after World War I.