Bird flu is very contagious among many bird species. In general, bird flu is not very contagious to humans, even to poultry workers. However, human-to-human spread has occurred in isolated cases. In human outbreaks, the first individual to become infected usually has had contact with infected birds or poultry and then caregivers become infected. Thus, caring for a person infected with bird flu is also a risk factor for the disease. There is a theoretical risk in laboratory workers who handle the avian flu virus. One alleged incident in 2009 occurred when a company inadvertently sent live avian flu virus samples to research laboratories, which subsequently were used to vaccinate ferrets. The contaminated vaccine did not result in any human infections.