In the mid-1950s he began experimenting with new photographic techniques to make extreme close-up photographs. These advances, combined with very thin endoscopes that became available in the mid-1960s, enabled him to make groundbreaking photographs of living human blood vessels and body cavities. He achieved international fame in 1965, when his photographs of the beginning of human life appeared on the cover and on sixteen pages of Life magazine. They were also published in Stern, Paris Match, The Sunday Times, and elsewhere. The photographs made up a part of the book, A Child is Born (1965); image from the book were reproduced in the April 30 1965 edition of Life, which sold eight million copies in the first four days after publication.
Nilsson has also been involved in the creation of documentaries, including The Saga of Life (1982) and The Miracle of Life (1996).