mit Langenscheidt:
1) that illness weighs heavily on my/ your/ someones mind
2) that "makes her melancholy"

...melancholic (not melancholy)

oder auch

The sickness makes him/her feel depressed.
The sickness makes him/her depressed.
The sickness depresses him/her.

as a more informal alternative

get a bad mood

schlägt auf may or may not convey a sense of something happening gradually or becoming worse over a span of time.

Takes a toll on conveys a sense of gradual decay or erosion occurring over time.

...takes a toll on one's disposition (or on someone)
...takes a toll on one's state of mind
...takes a toll on one's spirits (more literary)
...affects one's mood


Please disregard "get a bad mood". This is poor construction even in colloquial discourse.

Possible would be: "Get in or into a bad mood"

Better would be:

"Fall into a bad mood"
"someone is in a bad mood"
"(some occurrence) put someone in a bad mood"
"(some occurrence) left someone in a bad mood"

However, with get one can use this colloquial phrase, which refers to a temporary, ornery (unlikable, stubborn, difficult to deal with) state of mind:

"He got into a snit because the bartender wouldn't serve him another beer."
"Don't get into a snit about it. It's no big deal."
"Why do you have to get into such a snit about every little thing? Just chill out."

jemandem aufs Gemüt schlagen (deprimierend auf jemanden wirken)

(Duden)

Ich weiß daher nicht recht, was 'bad mood' hier zu suchen hat.
Am ehesten passt m.E. #3.

@Braunbärin

I just wanted to address #5, which will exist in this thread forever. People read through entire threads to learn not only how contributors dealt with the original question but also to pick up knowledge from everything else that's written as well. My intention was simply to point out how to better use "bad mood" in context. Do keep in mind that "Gemüt" is in the subject line, so anyone who looks up that word may well end up reading here.

mel·an·chol·y (mĕlən-kŏl′ē)

n.
1. Sadness or depression of the spirits; gloom.

2. Pensive reflection or contemplation.

3. Archaic
a. Black bile.

b. An emotional state characterized by sullenness and outbreaks of violent anger, believed to arise from an excess of black bile.

adj.
1. Feeling, showing, or expressing depression of the spirits; sad or dejected. See Synonyms at sad.

2. Causing or tending to cause sadness or gloom: a letter with some melancholy news.

3. Pensive; thoughtful.
https://www.ahdictionary.com/word/search.html...
(My bold)

I'm not sure that I understand the objection made in 2007 to "melancholy."

I also don't understand what "snit" has to do with "aufs Gemüt schlagen," or why it was originally marked as "Österreich."

I don't know what melancholy has anything more (or less) to do with this thread than snit does, but I do hope everyone learns a bit more about moods and states of mind here;-)
It's all good. How we learn is rarely linear. Main thing is that things are interesting enough to make us want to learn.

As Braunbärin has already pointed out:
Wendungen, Redensarten, Sprichwörter
•jemandem aufs Gemüt schlagen (deprimierend auf jemanden wirken)
http://www.duden.de/rechtschreibung/Gemuet

As the AHD tells us:
snit (snĭt)
n.
Informal
A state of agitation or irritation.
https://www.ahdictionary.com/word/search.html...

As a translation, melancholy - in addition to some of the other earlier suggestions - seems much more appropriate to me.