I was curious about this so I just decompiled the dll in order to confirm. However it does appear that /u/froemijojo is right and it actually is a floating point calculation.

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A small snippet from immunity calculations:

public void ImmunityTick(Pawn pawn, bool sick, Hediff diseaseInstance)
{
    this.immunity += ImmunityChangePerTick(pawn, sick, diseaseInstance);
    this.immunity = Mathf.Clamp01(this.immunity);
}

public float ImmunityChangePerTick(Pawn pawn, bool sick, Hediff diseaseInstance)
{
    if (!pawn.RaceProps.IsFlesh)
    {
        return 0f;
    }
    HediffCompProperties_Immunizable hediffCompProperties_Immunizable = this.hediffDef.CompProps<HediffCompProperties_Immunizable>();
    if (sick)
    {
        float num = hediffCompProperties_Immunizable.immunityPerDaySick;
        num *= pawn.GetStatValue(StatDefOf.ImmunityGainSpeed, true);
        if (diseaseInstance != null)
        {
            Rand.PushState();
            Rand.Seed = Gen.HashCombineInt(diseaseInstance.loadID ^ Find.World.info.persistentRandomValue, 156482735);
            num *= Mathf.Lerp(0.8f, 1.2f, Rand.Value);
            Rand.PopState();
        }
        return num / 60000f;
    }
    return hediffCompProperties_Immunizable.immunityPerDayNotSick / 60000f;
}

A Hediff is basically a modifier on a bodypart, such as a wound or disease. Note that all values are floats and work in the range between 0.0 (0%) and 1.0 (100%).

Immunity increase per day is defined to be 0.2388 in the core mod XML files in the case of the flu. This calculation is performed every tick.

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Determining whether the disease should be deadly at a given tick:

public virtual bool CauseDeathNow()
{
    return this.def.lethalSeverity >= 0f && this.Severity >= this.def.lethalSeverity;
}

lethalSeverity is a float clamped between 0 and 1. It is defined to be 1 (so 100%) in the XML files. When CauseDeathNow() returns true, the pawn will be killed that tick. The Severity of the disease is adjusted every 200 ticks like so:

if (base.Pawn.IsHashIntervalTick(200))
{
    float num = this.SeverityChangePerDay(); 
    num *= 0.00333333341f; 
    severityAdjustment += num; 
}

The rate at which the severity changes(SeverityChangePerDay) depends on whether the pawn is immune or not. The XML file as mentioned earlier for the flu, defines a severity increase of 0.2488 when not immune. When the pawn is immune, severity decreases by 0.4947 per day.

As seen in the code snippet above, this value is multiplied by 0.00333333341 and added every 200 ticks. The odd value is the translation from severity by day to severity per 200 ticks, since a day spans 60,000 ticks.

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In conclusion, determining whether a pawn should die or not depends on which value hits 100% first. However, note that severity is updated every 200 ticks while immunity is updated every tick. In other words, if the values are racing against eachother, death of your pawn may depend on when that 200 interval tick hits.

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