Arsenic makes mice more susceptible to H1N1 flu  virus.
http://www.environmentalhealthnews.org/ehs/newscience/low-dose-arsenic-alters-mouse-response-to-flu/
 http://www.environmentalhealthnews.org/ehs/newscience/low-dose-arsenic-alters-mouse-response-to-flu/
Kozul, CD, KH Ely, RI Enelow and JW Hamilton. 2009. 
 Low dose arsenic compromises the immune response to influenza A infection  in vivo. Environmental Health Perspectives doi:10.1289/ehp.0900911
 Low dose exposure to arsenic compromises the immune response to infection  in mice, increasing their vulnerability to the flu virus. By itself, the metal  altered the number and function of certain immune cells. The changes predisposed  the rodents to more severe reactions when exposed to the Influenza A  virus.