Il virus causa la febbre.

Because Italian nouns usually have grammatical gender and normally take an article.

• virus is masculine singular, so it takes il → il virus
• febbre is feminine singular, so it takes la → la febbre

These articles mean the in English.

A quick note:

• virus is one of those nouns that stays the same in the singular and plural form: il virus / i virus
• febbre changes in the plural: la febbre / le febbri

Italian often uses the definite article more than English does, especially with general nouns and many medical conditions.

So Italian commonly says:

• avere la febbre = to have a fever
• causare la febbre = to cause fever / to cause a fever

In English, the article may be omitted in some contexts, but in Italian it is very natural to keep it here.

Here, causa is a verb, from causare, meaning to cause.

So the sentence structure is:

• Il virus = the subject
• causa = the verb
• la febbre = the object

Be careful: causa can also be a noun in other contexts, meaning cause.
For example:

• La causa del problema = The cause of the problem

But in Il virus causa la febbre, it is clearly the verb.

Because causare is the infinitive, meaning to cause, while causa is the conjugated form used with he/she/it.

The subject is il virus, which is third-person singular (it), so the verb must match:

• causare = to cause
• io causo = I cause
• tu causi = you cause
• lui/lei/esso causa = he/she/it causes

So:

• Il virus causa la febbre = The virus causes fever

Because Italian does not need an extra helping verb here.

In English, we say:

• The virus causes the fever or in questions/negatives:
• Does the virus cause the fever?

In Italian, the main verb carries the meaning by itself:

• Il virus causa la febbre

There is no need for a separate word like does in a normal affirmative sentence.

Also, is would only be used if the sentence were about being, not causing.

Yes, here it is the standard word order:

• Il virus = subject
• causa = verb
• la febbre = object

So it follows Subject + Verb + Object, just like English:

• The virus causes fever

Italian can sometimes change word order for emphasis, but this sentence is neutral and standard.

A simple English-friendly guide is:

eel VEE-roos KAU-zah lah FEB-breh

A few pronunciation notes:

• il sounds like eel, but shorter
• virus in Italian is pronounced VEE-rus / VEE-roos, not like English VY-rus
• causa has au like in house or cow
• febbre has a doubled bb, so the b is held a little longer: FEB-breh
• The stress is:
  • VÍrus
  • càusa
  • fébbre

There is not always a simple logical rule for grammatical gender. You usually have to learn each noun with its article.

That said, many nouns ending in -e can be either masculine or feminine, so the ending alone does not tell you the gender.

Examples:

• il pane = bread (masculine)
• la febbre = fever (feminine)

So it is best to learn:

• la febbre, not just febbre

Not normally in standard Italian.

A more natural sentence would be:

• Un virus causa la febbre = A virus causes fever or
• Il virus causa la febbre = The virus causes fever

Leaving out the article before febbre sounds unnatural in this context. Italian usually wants the article there.

Yes. Provocare is another common verb meaning to cause / to provoke / to bring about.

So:

• Il virus causa la febbre
• Il virus provoca la febbre

Both are possible.
In medical or formal contexts, both sound natural, though there can be slight differences in tone depending on context. Causare is very direct and transparent for learners.

If you make both nouns plural, you get:

• I virus causano la febbre = Viruses cause fever

Notice:

• il → i
• virus stays virus
• causa → causano

If you wanted to pluralize fevers, it would be:

• le febbri

But in many contexts, Italian still keeps la febbre as a general condition rather than making it plural.

Yes. Virus is commonly invariable in Italian.

So:

• il virus
• i virus

This is different from many regular Italian nouns, which change ending in the plural.
For example:

• libro → libri
• casa → case

But virus usually stays the same.

Yes, it is grammatically correct and natural.

It sounds like a general scientific or medical statement:

• Il virus causa la febbre = The virus causes fever

Depending on context, Italians might also say:

• Il virus provoca la febbre
• Questo virus causa la febbre = This virus causes fever
• Il virus può causare la febbre = The virus can cause fever

But the original sentence is perfectly good Italian.