Adam Cross was a physical education teacher and varsity football coach at the local high-school, scoutmaster and volunteer firefighter from Wrigley.
Adam Cross was among a group of volunteer first responders who aided in the recovery after the Battle of New York. He and fellow firefighters found a Chitauri Helmet that they kept as a souvenir. Unbeknownst to them, the helmet carried a virus which causes those who contract the virus to die in a very strange way. Months after, Cross, Frank Whalen and Tony Diaz decided to polish the helmet. The helmet carried an alien virus, which Cross contracted.
Cross volunteered as a scout leader, and while out with the scouts, he began to feel the effects of the virus, and was eventually killed by it. The virus caused the body of the victim to float after death, which caused Phil Coulson and his team to investigate. While Jemma Simmons was investigating the body of Cross, a spark jumped from Cross to Simmons. Simmons did not even realize at this point that a virus had caused the death of Cross, much less that the virus traveled from body to body via the spark.