Learn Spanish (Spain): Ellos están enfermos y tienen fiebre.

Adjectives in Spanish must match the noun they describe in gender and number. Since ellos is a masculine plural group, the adjective takes the masculine plural ending -os.

The English translation says 'have a fever', but there is no una in the Spanish sentence. Why?

In Spanish, when talking about having common symptoms like a cold or a fever (fiebre), you usually drop the word for 'a' or 'an'. You simply say tienen fiebre.

Why isn't there a word for 'they' before tienen?

In Spanish, you do not need to repeat the subject pronoun. Since the sentence started with ellos, it is already clear who the subject is. The verb form tienen also specifically tells us the subject is 'they'.