Learn Spanish (Latin America): Ojalá que la fiebre sea el único síntoma.

Sea is the present subjunctive form of ser (to be). We use the subjunctive after ojalá (que) because it expresses a hope or a wish, rather than stating a guaranteed fact.

Do I always have to use que after ojalá?

No, the que is completely optional. You can say Ojalá la fiebre sea el único síntoma and it means exactly the same thing. Including que is just a very common habit in everyday spoken Spanish.

Why is it el único síntoma if the word síntoma ends in an 'a'?

Síntoma is a masculine noun, which is why it uses el and the masculine adjective único. Many Spanish words that end in -ma (like el problema, el sistema, and el tema) originally come from Greek and are masculine exceptions to the rule.

Since a fever is temporary, why do we use a form of ser (sea) instead of estar (esté)?