Learn Hardcore French: Heureusement, cette maladie rare avance lentement grâce au traitement. - Fortunately, this rare illness progresses slowly thanks to the treatment.

ce is masculine singular (before a consonant): ce livre
cet is masculine singular (before a vowel or silent h): cet homme, cet arbre
cette is feminine singular (before any letter): cette maladie, cette maison

So you must say cette maladie, never ce maladie or cet maladie.

Why is maladie rare and not rare maladie?

Most adjectives in French normally go after the noun, including rare in its basic, objective meaning (uncommon). So une maladie rare = a rare disease (not common).
Putting rare before the noun (une rare maladie) is possible, but it sounds literary and often adds a nuance of precious / exceptional in a positive or emotional sense, which would be odd when talking about an illness. In everyday French, for diseases you almost always say une maladie rare.

What exactly does avance mean here, and why use it for a disease?

Literally, avancer means to advance / to move forward. With illnesses, la maladie avance or la maladie progresse means the disease is progressing / spreading / getting worse over time.
You could also say:

cette maladie progresse lentement
cette maladie évolue lentement
All of these describe the course or progression of the disease. Avance is quite natural in medical or everyday speech.

Why is it avance lentement instead of est lente?

avance lentement focuses on the process: the disease progresses slowly (how it moves forward in time).
est lente would describe a static characteristic: the disease is slow.
French usually talks about the course of a disease with verbs like avancer, progresser, évoluer, followed by an adverb (lentement, rapidement). So avance lentement sounds more natural and more precise.

Why is lentement (adverb) used instead of lent (adjective)?

In French, adjectives modify nouns, and adverbs modify verbs, adjectives, or other adverbs.

lent is an adjective → un train lent (a slow train).
lentement is the adverb → it tells you how the verb action happens: avance lentement (advances slowly).
Since lentement modifies the verb avance, the adverb form is required.

What does heureusement mean here, and why is there a comma?

Heureusement means fortunately / luckily. At the beginning of a sentence, it’s a sentence adverb, commenting on the whole idea: Fortunately, this rare disease progresses slowly….
The comma separates this comment from the main statement. You could also place it elsewhere for nuance:

Cette maladie rare, heureusement, avance lentement… (more spoken style)
But the typical neutral order is exactly what you see.

Can I say Heureusement que cette maladie rare avance lentement… instead?

Yes. Heureusement que… is also very common and natural.
Subtle differences:

Heureusement, cette maladie rare avance lentement… = more neutral, a simple comment.
Heureusement que cette maladie rare avance lentement… often feels a bit more emotional or conversational, like Thank goodness this disease progresses slowly….
Both are correct; the original sentence is slightly more formal/neutral.

What is grâce au traitement exactly, and why not just à cause du traitement?

Grâce à expresses a positive cause: thanks to, because of (in a good way). Here, the treatment has a beneficial effect (the disease advances slowly), so grâce à is appropriate.
À cause de is typically negative: because of (in a bad way).

La maladie avance rapidement à cause du traitement. = The treatment makes things worse.
So grâce au traitement clearly says the treatment is helping.

What does au stand for in grâce au traitement?

Au is the contraction of à + le:

grâce à
- le traitement → grâce au traitement.
  French always contracts à + le → au, and de + le → du. You can’t say à le traitement in standard French.

Is there any agreement to worry about in cette maladie rare avance lentement?

Yes, with the noun phrase:

cette agrees with maladie (feminine singular).
rare also agrees with maladie, so it takes the feminine singular form rare (same spelling as masculine, but it’s still grammatically feminine).
The verb avance agrees with the subject cette maladie rare (third person singular).
Lentement is an adverb, so it never changes form; it stays the same regardless of gender or number.

Could we say Heureusement, la maladie rare avance lentement… instead of cette maladie rare?

You could, but it changes the reference:

cette maladie rare = this rare disease (the one we’re currently talking about, more specific and demonstrative).
la maladie rare would sound like the rare disease in a more general or context-assumed way, and in many contexts it would feel less natural.
If the disease has just been introduced or is being pointed out specifically, cette maladie rare is better.

Is the word order cette maladie rare avance lentement fixed, or could we move things around?

French word order is relatively fixed:

Normal structure: Subject + Verb + (Adverb) → Cette maladie rare avance lentement.
You generally can’t put the full subject after the verb in a neutral statement (like English “Advances slowly this rare disease”). That would sound poetic or archaic in French.
You can, however, move some sentence adverbs:
Heureusement, cette maladie rare avance lentement.
Cette maladie rare avance, heureusement, lentement. (more oral / expressive)

Could I replace avance with progresse, évolue, or empire?

Yes, but the meaning shifts slightly:

progresse = progresses (quite close to avance, often used with diseases).
évolue = evolves / changes over time (more neutral, could be better or worse).
empire = gets worse.
So:
cette maladie rare progresse lentement ≈ similar to the original.
cette maladie rare évolue lentement = it changes slowly (not clearly good or bad).
cette maladie rare empire lentement = it worsens slowly (clearly negative).
The original just says it is moving forward slowly, without specifying good or bad beyond the context.

How is heureusement pronounced, and are there any tricky silent letters in the sentence?

Key points:

Heureusement: roughly /œʀøzəmɑ̃/ (in many accents). Final -t is silent.
cette: final -e is pronounced (short “e”), final -t is pronounced.
maladie: final -e is pronounced, no liaison with rare afterward.
rare: final -e is pronounced, final -r is usually pronounced softly.
avance: final -e is silent, -ce = /s/.
lentement: lent alone is /lɑ̃/ (final -t silent), but in lentement, you do hear a t sound in the middle: /lɑ̃təmɑ̃/. Final -t is silent.
traitement: /tʀɛtmɑ̃/, final -t silent.