Many infested have developed deadly venoms and poisonous claws, as a result their attacks stand a chance of giving you a minor infection. While 1 is minor more of them can mean the Marines slow and painless death.
Parasites appear rarely, but should always be killed immediately. They possess one of the lowest threat levels of any enemy. Parasites will spawn from the corpses of Tartarus, Agrons, Infestors, the Zombie Queen, and Infested Worms on higher difficulties.
Lelanthos' primary attack is a pair of Broodlings. Although not particularly strong, they have a very fast attack that has a high chance to inflict the player with Venom.
Venom only lasts three minutes, so sometimes in situations where a player has many venoms, it is more energy efficient to keep them sufficiently healed, rather than try and cure all the venoms themselves.
Quite common, and available in packs of 5, anti-venom is a good way to cure venom. Most players prefer to save their anti-venom for situations where a Medic may be unable to heal every character, or is drained of energy. Nonetheless this is possibly the most convenient way to cure the ailment.
An innate skill that every Medic and Chemical Expert will have, the Anti-Venom skill does have peculiar hotkey attached to it. It is found under Marine Actions, so the full hotkey is X then V. Many new players will not know this, and take a long time to figure out on their own. Either way, this skill has a short cooldown, low energy cost, and can be used many times if the Medic/Chemical Expert has a healthy supply of energy. If a medic has the restore skill they can use it, then the AV skill to cure all venom stacks on a friendly marine.
Riflemen with Saline IV upgraded to level two or higher can cure one venom every usage. The skill has a modest energy cost, and a somewhat long cooldown. However, it also heals a considerable amount of health, this is usually sufficient to keep the rifleman from dying until he receives better aid.