decease & disease

Why they are so similar?
I had confused them already.
I didn't even know about the existence of first one.

See a translation

They do sound quite similar, but the giveaway difference is how you pronounce the C and the S'es.

Both the C and the S in "decease" are pronounced as S'es, Duh-SEES. You also say the "sees" for the same length of time as you say the "duh".
Duh-SEES.

The S'es in "disease" are pronounced as Z's. Duh-ZEEZ. You also say the "zeez" for about twice as long as you say the "duh".
So it's like Duh-ZEEEEZ.

Duh-SEES vs. Duh-ZEEEEZ.

I can't record myself saying them, but put them both into the Google Translate text-to-voice and pay attention to that, and you should notice the difference.

If you don't already know the difference in meaning, "decease" means the same thing as "die", and a disease is a kind of illness.