Нужно бороться с болезнью.

Нужно means it is necessary, one must, or need to, depending on context.

This sentence is an impersonal construction, which is very common in Russian. Russian often does not use a subject when the meaning is general:

• Нужно бороться с болезнью. = It is necessary to fight the disease / One must fight the disease.

If you want to say who needs to do it, Russian usually adds that person in the dative:

• Мне нужно бороться с болезнью. = I need to fight the disease.
• Нам нужно бороться с болезнью. = We need to fight the disease.

After нужно, Russian normally uses the infinitive to say what is necessary.

So the pattern is:

• нужно + infinitive

Examples:

• Нужно работать. = It is necessary to work.
• Нужно учиться. = It is necessary to study.
• Нужно бороться с болезнью. = It is necessary to fight the disease.

A finite verb like борюсь or боремся would only be used if you explicitly state who is doing the action:

• Я борюсь с болезнью. = I am fighting the disease.

Бороться is the normal dictionary form of the verb meaning to fight, to struggle, or to battle.

The ending -ся is often called reflexive, but in many verbs it does not literally mean someone is doing the action to themselves. Here, бороться does not mean fight oneself.

It is simply the standard verb used in this pattern:

• бороться с кем-то / чем-то = to fight / struggle against someone or something

So in this sentence, just learn бороться с + instrumental as a unit.

Because the verb бороться requires the preposition с and the noun after it goes into the instrumental case.

So:

• болезнь = nominative
• болезнью = instrumental

This is the standard pattern:

• бороться с врагом = to fight the enemy
• бороться с проблемой = to fight a problem
• бороться с болезнью = to fight an illness/disease

Yes, с literally often means with, but after бороться it is translated more naturally as against or with in the English sense of struggle with.

So:

• бороться с болезнью literally looks like fight with the disease
• but natural English is usually fight the disease, fight against the disease, or struggle with the disease

This is one of those verb-preposition combinations that should be learned together.

Бороться is imperfective, so it focuses on the process, effort, or ongoing struggle.

That fits well with a general statement like:

• Нужно бороться с болезнью. = One must fight the disease.

The sentence is about the need to engage in the struggle, not about the final result.

If you want to emphasize the result, Russian often uses a different verb:

• побороть болезнь = to overcome the disease
• победить болезнь = to defeat the disease

So the contrast is roughly:

• бороться с болезнью = to fight/struggle against the disease
• побороть / победить болезнь = to defeat / overcome the disease

Yes. Надо бороться с болезнью is perfectly natural and very close in meaning.

The difference is mostly stylistic:

• нужно = neutral, common, slightly more formal or matter-of-fact
• надо = also very common, often a bit more conversational

In many situations, they are interchangeable.

No, Russian word order is fairly flexible.

The neutral order here is:

• Нужно бороться с болезнью.

But you can also say:

• С болезнью нужно бороться.

This version puts more emphasis on с болезнью — as if contrasting it with something else, or highlighting what must be fought.

Russian word order often changes focus or emphasis, not the basic grammatical meaning.

It can be either, depending on context, because Russian has no articles.

So с болезнью could mean:

• with the disease if a particular illness is already known from context
• with illness or with disease in a more general sense

If you wanted a clearly plural idea, you could say:

• с болезнями = with diseases

But singular nouns in Russian are often used in a broad or abstract way too.

The stress is:

• Ну́жно боро́ться с боле́знью.

A rough pronunciation guide:

• Нужно ≈ NOOZH-na
• бороться ≈ ba-ROTs-ya
• с болезнью ≈ z ba-LYEZN'-yu or s ba-LYEZN'-yu depending on speed and assimilation

A couple of useful notes:

• In fast speech, с болезнью may sound a bit like з болезнью because the с is influenced by the following voiced consonant.
• The -ться in бороться is usually pronounced roughly like -ца in normal speech.

So the whole sentence sounds approximately like:

• NOOZH-na ba-ROTs-ya z ba-LYEZN'-yu