The Infected Venom are special Infected.
"This corrupt swamp creature can spit its infected and acid saliva causing damage to several units at a time."
One of the most dangerous Infected, Infected Venom can attack towers and units from range, thus having the ability to neutralize colony defenses. Their acidic projectiles deal damage in moderate AoE, but take time to reach the target and it is possible to move units out of the way to avoid damage. Snipers are best equipped to deal with them as they have superior range.
Some maps in campaign feature big clumps of Venoms. On 800% difficulty there can be hundreds of Venoms in single area. If multiple Venoms start firing together, they may kill a group of units in single barrage. What's worse, they are often accompanied with Harpies that draw fire to them, and reinforcing Venoms may interrupt regeneration of units. Caution should be practiced not to pull more Venoms than colony's units can handle. Long-ranged attack with protections, like snipers on towers, are recommended. You may also need a group of Thanatos or Titans to take out them from afar. Towers are also good choices to absorb damage from them.
Survival maps don't have crowds of Venoms, but significant numbers exist on edges of the map and arrive during the final wave. They may infect defensive towers and kill defenders long before the walls fall. To prevent this, make sure outer layer of walls is at least 5 tiles away from nearest tower or unit.