लक्षण संक्रमण की वजह से है।

A simple breakdown is:

• लक्षण = symptom / sign
• संक्रमण = infection
• की वजह से = because of / due to
• है = is

So the whole sentence means The symptom is due to an infection or The symptom is because of the infection, depending on context.

Because वजह is a feminine noun.

In Hindi, का / की / के agrees with the noun that follows it in this kind of structure. Here that noun is वजह, which is feminine singular, so Hindi uses की:

• संक्रमण की वजह = the reason/cause of the infection
• संक्रमण की वजह से = because of the infection

So की is there because of वजह, not because संक्रमण is feminine.

It is a very common expression meaning because of, due to, or as a result of.

You can think of it literally as something like:

• X की वजह = the reason of X
• X की वजह से = because of X

So:

• बारिश की वजह से = because of the rain
• ट्रैफ़िक की वजह से = because of the traffic
• संक्रमण की वजह से = because of the infection

It is best learned as a set phrase.

Because many Hindi nouns, especially Sanskrit-based nouns ending in a consonant, do not visibly change in this position.

So even though संक्रमण is connected to की वजह, its form stays संक्रमण.

Compare that with a noun that does change:

• लड़का → लड़के की वजह से

But with संक्रमण, the form remains the same:

• संक्रमण की वजह से

Here it is singular, because the verb is है.

• लक्षण ... है = the symptom is ...
• लक्षण ... हैं = the symptoms are ...

A tricky point is that लक्षण can look the same in singular and plural in the direct form. So the verb helps you know the number.

If the sentence were plural, it would be:

• लक्षण संक्रमण की वजह से हैं। = The symptoms are due to the infection.

That is normal Hindi word order.

Hindi often puts the main information first and the verb or copula at the end. So the structure here is roughly:

• लक्षण = subject
• संक्रमण की वजह से = cause phrase
• है = is

A very literal English-style ordering would sound like:

• The symptom due to infection is

That sounds strange in English, but it is normal in Hindi.

Because Hindi usually does not use articles like English a/an/the.

So:

• लक्षण can mean a symptom, the symptom, or just symptom
• संक्रमण can mean an infection, the infection, or just infection

Context tells you which one is meant.

In this sentence, से is part of the fixed expression की वजह से.

On its own, से often means things like from, with, by, or than, but in this phrase it helps create the meaning because of or due to.

So you should understand:

• वजह = reason/cause
• वजह से = because of / due to

Without से, the sentence would not express the same complete idea.

Yes. A common alternative is:

• लक्षण संक्रमण के कारण है।

Here के कारण also means because of / due to. This version sounds a bit more formal or written.

You could also say:

• यह लक्षण संक्रमण की वजह से है।

Adding यह makes it This symptom is due to the infection.

Yes, it is grammatical and understandable.

In real conversation, a speaker might often add a demonstrative if the symptom has already been identified:

• यह लक्षण संक्रमण की वजह से है।

Or in a more formal/medical style:

• यह लक्षण संक्रमण के कारण है।

But the original sentence is still a valid Hindi sentence.