Its calculated as per the final paragraph on page 365. Remember that damage can be inflicted on each combat action available to the swarm, on a successful attack. Swarms are nasty because its impossible to use Special Effects against them: fortunately, they can't use any back.
So a swarm of SIZ 20 has the ability to inflict up to 4d4 damage in a round - an average of 10 points of damage. On the next round, this stacks to an average of 20. The character is therefore making an Endurance roll against 20 from round 2, 30 from round 3 and so on. Larger swarms are more potent and so the toxicity will mount quicker.
If you're a particularly nasty GM you could rule that insects getting inside armour are able to sting at maximum damage. So you might make one (or more) of the damage dice its maximum value again, increasing toxicity.
Anaphylactic shock? If an Endurance roll is fumbled while resisting, the victim suffers the Asphyxiation condition as outlined on page 113.