The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

We used to, but a couple guys
in the INFECTIOUS disease lab
The Big Bang Theory Season 12, Episode 6

This chamber is needed
for the INFECTIOUS,
Game of Thrones Season 7, Episode 3