You have to place the resource you gain from Viral Enhancer on the card just played, so when playing a Plant-tag, you normaly have no choice then to take a Plant-resource, as there is no card that collects plants on it. With the Microbe- and Animal-tag you gain a corresponding resource, as I could not find a card that has microbe-tag and collects animals and vice versa.
But you are always free to take a plant when playing a Microbe- or Animal-tag.
Hope this helps.
Dirk is right:
When you play a card with plant/microbe/animal tags, you choose:
- put a resource cube on the card you just played (if possible)
- or take a plant resource on your player board
Each tag counts, so if you play Advanced Ecosystems, Viral enhancer would let you take 3 plant resources (one for each tag). But since Adv. Ecosystems is not a card that collects resources you cannot but anything on it.
The thought behind Viral Enhancer was simple enough, but it was not easy to formulate the effect in an easy way!