Half of all new HIV transmissions occur when people are unlikely to know they carry the virus and in some cases, wouldn't test positive for it because they are so newly infected, according to a new study authored by Mark Wainberg, director of the McGill AIDS Centre. The study is to be published in the April edition of the Journal of Infectious Diseases and is one of the first in the world to quantify how many of the newly infected are responsible for spreading the disease to others.