This should sound somewhat familiar: a small band of mercenaries are on a forbidding planet, confronted by a horrific alien force that swarms across the landscape. Well-armed and entrenched, the soldiers have been easily keeping the enemy at bay despite being surrounded when, suddenly, the air becomes thick and acrid. Coughing and sputtering, the squad slowly begins to weaken from poison in the air while the aliens keep coming in force and threaten to overrun them.
That's the experience of playing Infested Planet. No matter how strong your defense or capable your soldiers, death is always just one mutation away.