Lister left school in the spring of 1844 when he was seventeen. He was unable to attend either Oxford or the University of Cambridge owing to the religious tests that effectively barred him. Lister decided to attend the non-sectarian University College London Medical School, one of only a few institutions which accepted Quakers at that time. He initially studied arts, graduating with a Bachelor of Arts degree with honours in classics and botany in 1847. While he was studying, Lister suffered from a bout of smallpox, a year after his elder brother died of the disease. The bereavement combined with the stress of his classes led to a nervous breakdown. Lister decided to take a long holiday in Ireland, to recuperate and this delayed the start of his medical studies at the university. In October 1848, Lister registered as a medical student. While a student, Lister developed a lifelong interest in histology and experimental physiology. During his studies, Lister was active in the University Debating Society and the Hospital Medical Society. His main lecturers were John Lindley, Thomas Graham, Robert Edmond Grant, George Viner Ellis and William Benjamin Carpenter but although Lister often spoke about Lindley and Graham in his writings, it was Wharton Jones and William Sharpey who exercised the greatest influence on Lister. As part of his studies, Lister trained first as an intern and then house physician to Walter Hayle Walshe and then 1851, house surgeon to John Eric Erichsen.