Viruses, Bacteria, Fungus, yeasts and Food
I agree viruses are likely not a worry.
Bacteria, Fungus and Yeasts could be. Maybe even Algae could be dangerous. Some of them are not species specific and others are.
There is no reason to suppose that Plant life elsewhere is all inedible. It will have Starches /Carbohydrates/sugars/minerals. Some may have poisonous chemicals (like oxalic acid). Vitamins may be an issue. We can of course plant our own seeds and roots etc. But better be careful.