Dr. Jean Lindenmann, a co-discoverer of interferon, the powerful antiviral substance used to treat some cancers as well as hepatitis C and multiple sclerosis, died on Jan. 15 in hospice care in Zurich. He was 90.

The cause was complications of prostate cancer, his son Christian said.

A Swiss scientist, Dr. Lindenmann made his discovery in 1957 when he was a postdoctoral student at the National Institute for Medical Research in London, working with Dr. Alick Isaacs.

The two began studying a puzzling phenomenon: If they killed viruses by heating them and then added those dead viruses to cells, the cells resisted subsequent infection with live viruses. Was it because the dead viruses tied up entry portals in the cells, preventing live viruses from getting in? Were the dead viruses secreting something that acted like a viral version of an antibiotic?

The answer, they discovered, was neither. It turned out that the dead viruses prompted the cells to resist infection by secreting a substance the scientists named interferon, because it interfered.