Disease and toxins attack the body every bit as much as weapon strikes, and the mechanic is remarkably similar. The main difference is that disease and toxins have a wider variety of possible Bad Outcomes than say a hatchet.
Any disease or toxin should be assigned a dice value. That value is as much about the needs of the plot as it is about specific research. Most naturally occurring diseases and toxins cap out at 9 dice. Any disease or toxin under 3 dice is likely not considered fatal except to the very weak.
With the onset of the disease or toxin (however that happens) the affliction rolls its dice like an attack, and the victim rolls dice+EN (plus SZ in the case of toxins) against it. The remainder is divided by the Damage Divisor as in a normal attack, to determine damage. The affliction could also have a Restraining effect (from muscle cramps or nerve dysfunction) on top of the damage.
In general, toxins only get the one attack roll. It either kills you or it doesn't.
Disease, in contrast, keeps attacking, though it loses one dice with each successive attack. Unless otherwise indicated, these attacks occur daily until the disease runs out of dice or the victim dies.
Any character that survives a toxin will gain +1 (as if a skill) vs that same toxin in the future. Any character that survives a disease gains an extra dice vs that same disease in the case of future infections.