Humans may get bird flu from contact with infected birds (chickens, for example) or their infected droppings and secretions. Risk factors include caring for sick birds, killing sick birds, and preparing sick birds for consumption. Despite the large number of people who have contact with poultry every day in the world, human cases of bird flu remain rare. This highlights how difficult it is for the bird flu virus to infect human cells, but mutations like antigenic shifts may reduce such difficulties. The H1N1 pandemic that started in Mexico is an example of such a mutation (swine flu to human flu). Although direct contact with sick poultry poses the highest risk for bird flu, indirect exposure to bird feces or other materials such as bird eggs is also a risk. Contact with unwashed eggs from sick birds or water contaminated by poultry feces poses a potential risk of disease.