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Each symptom contributes differently to a survivor’s total infection probability. For example, green blood is more "suspicious" than red blood, but this alone cannot 100% confirm an infection. You’ll need to compare multiple vitals before deciding whether someone is infected.
So for example, a survivor with double irises, green blood, a fever, and an elevated heart rate is certainly infected. However, a survivor with green blood but otherwise normal vitals will only have a slightly increased infection probability.