Simple: To research Airborne Pathogen, you must first research Contagion I. You cannot begin researching Airborne Pathogen until Contagion I is finished.
Moderate: To Reanimate an enemy unit, you must research Infection I, then Suppression, then kill an enemy unit, then morph a Festering Corpse into the suppressed unit.
Advanced: There are several ways to create a Festering Corpse. One is to kill your own Abominations: First research Infection I. Then have a Mosquito attack a Maggot or another Mosquito, which will cause it to morph into an Abomination. Have the Mosquito attack that Abomination to give it a Carrier attachment. Once the Carrier card is face-up, kill the abomination with Infection I to turn it into a Festering Corpse.