In 1869 P. Pius IX (1846-1878) commissioned Roman architect Filippo Martinucci to rebuild this chapel in honor of San Sebastiano, San Rocco and San Marta, as protection against plague and infectious diseases. The altarpiece is Execution of St. Sebastiano (1614) by Giovanni de Vecchi. The painting shows that it had sustained some damage in the past.

(Another photo of altarpiece on the web here)