I was just watching something on National Geographic before and got an idea for skulks. the aliens obviously aren't the most hygienic bunch, and I was thinking that bacteria would build up in the mouths of the skulks and that an untreated bite could get infected and slowly make health drop, or lower the speed of the marine infected. The Tyrannosaurus Rex is speculated to have used this type of "biological weapon", an enemy or prey would die soon from a wound inflicted because of the bacteria from the T-rex's mouth. The infection could be healed, but the marine with it would have to make it to an armory or receive a med pack. The wound would become infected if it is not healed within say 25 - 30 seconds. Maybe Heavy armor could stop the marine from becoming infected, like spores can not damage a Heavy. Perhaps a marine who was being digested would get sick depending on how long they were in the Onos' stomach.