Fungi

There are exist a lot of fungi with many different properties.
Some fungi are poisonous and some have spores which can be bad for your health when you get them into your lungs.
Some fungi can even work as a huge network among an entire forest and can become several kilometers/miles large.

If you do some fancy combinations, you could get a usually harmless fungus which covers huge portions of a continent, but with a season where it distributes its spores (e.g. in autumn).
The wildlife could have adopted to the spores, but the spores could be harmful for humans, especially in big amounts.

One bonus effect: The continent could be habitable most time of the year, but not for a permanent stay.