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.

...that I am putting an end to your
PARASITIC piggybacking upon our Wi-Fi.
The Big Bang Theory Season 2, Episode 7

Parasitic infections are common,
such as pinworms.
The Big Bang Theory Season 5, Episode 1