The Chitauri Helmet is a component of the armor worn by the Chitauri soldiers as they invaded Earth during the Battle of New York. One such helmet was found by volunteer firefighters after the battle.
After the Chitauri forces were defeated by the Avengers, three volunteer firefighters named Adam Cross, Frank Whalen, and Tony Diaz helped in the clean-up efforts after the Battle. During the clean-up they found the helmet of one of the fallen Chitauri and decided to take it back to their fire station as a souvenir.
A few months later, when they started to clean what they thought was rust, they liberated a virus carried by the wearer of the helmet, that sequentially killed Cross, Whalen and Diaz, and infected Agent Jemma Simmons of S.H.I.E.L.D.