Если выйти без шапки и шарфа, можно быстро простудиться.

Because если выйти is an impersonal, general-condition pattern. It means something like:

• if one goes out
• if you go out
• if a person goes out

So the sentence is not talking to one specific person directly. It states a general truth.

Compare:

• Если выйти без шапки и шарфа, можно быстро простудиться.
  = If you go out without a hat and scarf, you can quickly catch a cold.

• Если выйдешь без шапки и шарфа, быстро простудишься.
  = If you go out without a hat and scarf, you will quickly catch a cold.

The version with выйти sounds more general and less personal.

There is no explicit subject, and that is normal in Russian.

The sentence means something like:

• anyone
• a person
• you in the general sense
• one

So выйти and простудиться here are understood as referring to people in general.

This kind of subjectless wording is very common in Russian when giving:

• general advice
• warnings
• common truths
• rules of life

Because they come after без, and без requires the genitive case.

So:

• шапка → без шапки
• шарф → без шарфа

This is a very common pattern:

• без сахара = without sugar
• без воды = without water
• без куртки = without a jacket

So in this sentence:

• без шапки и шарфа = without a hat and scarf

In this context, шапка usually means a warm hat, especially a winter hat. It is often broader and more practical than the English word hat, which can sound more like a fashion item.

So без шапки и шарфа strongly suggests:

• without proper cold-weather clothing
• without a winter hat and scarf

That is why the sentence sounds very natural in Russian as a warning about getting sick from cold weather.

Here можно means it is possible or one can.

So:

• можно быстро простудиться = you can quickly catch a cold / it’s easy to catch a cold quickly

This does not mean permission here. It is not you are allowed to catch a cold. It expresses possibility/risk.

Russian можно can mean different things depending on context:

• Можно войти? = May I come in?
• Здесь можно простудиться. = You can catch a cold here.

In your sentence, it clearly means possibility.

Простудиться is perfective, so it focuses on the result: getting a cold.

That fits well here, because the sentence is about what may happen as a consequence:

• go out without warm clothes → catch a cold

By contrast, простужаться is imperfective and would usually describe:

• repeated action
• a general tendency
• the process in a broader sense

So можно быстро простудиться means:

• you may end up catching a cold quickly

The perfective verb is the natural choice because the sentence points to a single result.

Быстро means quickly, and it modifies простудиться.

So the idea is:

• you can quickly catch a cold
• it can happen fast

It does not mean that the act of going outside is quick. It describes how quickly the bad result may happen.

Russian adverbs like быстро are fairly mobile, so you may also hear slightly different word orders, but here this placement is completely natural:

• можно быстро простудиться

Выйти is perfective and means to go out / step outside as a complete action.

In this sentence, the idea is:

• if you step outside / if you go out without a hat and scarf

Russian often uses the perfective infinitive in this kind of conditional-general structure when referring to a concrete action as a whole.

If you used выходить, it would sound more like repeated or habitual going out, and it would be less natural in this exact sentence.

So если выйти presents the action as a complete event: if one goes out.

Because the sentence begins with a subordinate clause introduced by если:

• Если выйти без шапки и шарфа, ...

Then comes the main clause:

• можно быстро простудиться

Russian normally separates an если clause from the main clause with a comma.

So the structure is:

• Если ... , ...

Just like:

• Если будет холодно, останемся дома.
• Если не учить слова, трудно говорить.

Yes. Russian word order is flexible, and you could also say:

• Можно быстро простудиться, если выйти без шапки и шарфа.

This means the same thing.

The difference is mainly in focus:

• Если выйти без шапки и шарфа, можно быстро простудиться.
  Starts with the condition.

• Можно быстро простудиться, если выйти без шапки и шарфа.
  Starts with the result/warning.

Both are natural.

It is neutral and natural. It could appear in:

• everyday speech
• parental advice
• a warning
• informal writing
• general health advice

It does not sound especially formal or slangy. It is a very standard Russian way to express a general warning.

A parent might say something more directly to a child, for example:

• Не выходи без шапки и шарфа — простудишься.

But your sentence is the more general, impersonal version.