A Chitauri helmet salvaged from New York City following an attempted alien invasion was kept as a souvenir by a team of Wrigley, Pennsylvania firefighters including Adam Cross and Tony Diaz. It carried a dormant alien virus which, when reactivated, proved life-threatening to those around. Only one infected party, Jemma Simmons, survived contracting the virus, and she and Fitz were able to concoct an anti-serum (or vaccine — the jury seems to be still out) using cells taken from the helmet from the original Chitauri soldier that wore it, who had a natural immunity to the virus.
Unlike Earthly viruses which are usually spread through fluids, the air, or sexual contact this virus spreads via electro-static shock. The virus gestates insides its host for about 36 hours before symptoms appear, which include hearing a humming sound and metal objects near the host floating caused by the electromagnetic field the host's body will generate. Finally, electricity continues to build in the host's brain until the electricity discharges, frying the brain, and releases a powerful electromagnetic pulse, killing the host. Furthermore, the virus alters its host's molecular density and polarity so that their body floats suspended in the air for some time after they die.