Learn French: J'ai peur que ce symptôme respiratoire soit grave, voire mortel. - I am afraid that this respiratory symptom is serious, and even deadly.

Voire is a coordinating conjunction used specifically to introduce a stronger, more extreme, or escalated idea. Here, it takes the severity of the symptom from grave (serious) to the next level: mortel (deadly). While you could use et même, voire is a very natural and slightly more elevated way to express this kind of "escalation" or "if not" progression in French.

You sometimes hear people say voire même. Would that be correct in this sentence?

You will hear native speakers say voire même mortel quite often in spoken French. However, strictly speaking, it is a redundancy (a pleonasm) because voire already carries the meaning of "and even". In written or careful French, it is always better to stick to just voire, exactly as it is used in this sentence.

Why is the verb être in the subjunctive form (soit) instead of the indicative (est)?

The phrase J'ai peur que (I am afraid that) expresses a strong emotion and a sense of uncertainty or fear. In French, expressions of emotion, doubt, or fear followed by que require the verb in the dependent clause to be in the subjunctive mood. That is why we use soit here.

The word symptôme ends in an "e". How can I remember that it is masculine, taking ce instead of ?