Doktor misli da je to samo virus, a ne nešto ozbiljnije.

Croatian does not have articles, so there is no direct equivalent of the or a/an.

That means:

• doktor can mean the doctor or a doctor
• virus can mean a virus or the virus

The exact meaning comes from context. In this sentence, English would most naturally say The doctor thinks it's just a virus... because we are probably talking about a specific doctor.

After verbs like misliti (to think), Croatian usually introduces the next clause with da.

So:

• misli = thinks
• misli da... = thinks that...

English often drops that, but Croatian normally keeps da.

So Doktor misli da je to... literally means The doctor thinks that it is...

Because je is a clitic. Clitics are short, unstressed words that usually go in the second position of their clause.

So Croatian prefers:

• da je to samo virus

not:

• da to je samo virus

This word order is very important in Croatian and is one of the things English speakers often have to get used to.

Here to is a neuter pronoun meaning something like that or it.

It refers to the situation being discussed — for example, the illness, symptoms, or problem.

So je to samo virus is basically it is just a virus.

Samo means only or just.

It is placed before the thing it limits or emphasizes:

• samo virus = just a virus

That placement is very natural in Croatian. It tells you that the doctor thinks the problem is nothing more than a virus.

In Croatian, a often introduces a contrast. It can mean something like while, whereas, or and/but depending on context.

In this sentence:

• a ne nešto ozbiljnije

means:

• and not something more serious
• or more naturally in English, not something more serious

Using a ne after the first part is very natural when contrasting two possibilities:

• samo virus
• ne nešto ozbiljnije

Ali is also a contrast word, but a is especially common in this kind of structure.

Virus is in the nominative singular.

That is because after biti (to be), Croatian often uses the nominative for the noun that identifies something:

• To je virus. = It is a virus.

So in da je to samo virus, the noun after je stays in the nominative.

Ozbiljnije is the comparative form of ozbiljan (serious).

The comparative forms are:

• ozbiljniji = masculine
• ozbiljnija = feminine
• ozbiljnije = neuter

Here it is neuter singular, matching nešto, which is also neuter.

So:

• nešto ozbiljnije = something more serious

Because the sentence is making a comparison.

The idea is:

• it is just a virus
• and not something more serious

So Croatian uses the comparative ozbiljnije to show a contrast with the first idea. The meaning is basically more serious than a virus or more serious than that.

If you used ozbiljno, it would mean serious, but the sentence here specifically contrasts one diagnosis with a worse one.

Because nešto already means something, and Croatian often leaves the noun unstated when it is obvious or unimportant.

So:

• nešto ozbiljnije = something more serious

It does not need an extra word like stvar, problem, or bolest unless the speaker wants to be more specific.

Yes, in standard written Croatian, the comma here is expected.

The sentence has two contrasted parts:

• Doktor misli da je to samo virus
• a ne nešto ozbiljnije

The comma helps mark that contrast clearly.

Yes.

Both can mean doctor in the medical sense:

• doktor = very common in everyday speech
• liječnik = also correct, sometimes felt as a bit more formal or specifically medical

So you could also say:

• Liječnik misli da je to samo virus, a ne nešto ozbiljnije.

Both are natural.