Wegen des Fiebers schicke ich heute eine Krankmeldung, und morgen hole ich die Krankschreibung ab.

In standard German, wegen normally takes the genitive case. That is why das Fieber becomes des Fiebers.

• das Fieber = nominative
• wegen des Fiebers = genitive after wegen

A very common colloquial alternative is wegen dem Fieber, but learners should usually treat wegen + genitive as the safest standard choice, especially in writing.

Because this is the genitive singular form of a neuter noun.

For many masculine and neuter nouns, the genitive singular adds -s or -es:

• das Fieber → des Fiebers
• das Kind → des Kindes
• der Arzt → des Arztes

So des Fiebers is just the normal genitive form of das Fieber.

German main clauses follow the verb-second rule. That means the finite verb must be in the second position.

In the first clause, Wegen des Fiebers comes first, so the verb has to come next:

• Wegen des Fiebers | schicke | ich heute eine Krankmeldung.

In the second clause, morgen comes first, so again the verb comes second:

• morgen | hole | ich die Krankschreibung ab.

If the subject came first, then you would get:

• Ich schicke heute eine Krankmeldung.
• Ich hole morgen die Krankschreibung ab.

They are related, but not the same.

• Krankmeldung = the act of reporting that you are sick, or the message/notice you send to your employer or school
• Krankschreibung = being officially signed off sick by a doctor; in everyday use, it can also refer to the doctor’s certificate

So the sentence suggests this sequence:

1. Today, the speaker sends a sickness notification.
2. Tomorrow, the speaker picks up the doctor’s note / official sick-leave certificate.

In very formal German, the certificate is often called an Arbeitsunfähigkeitsbescheinigung, but everyday speech often uses Krankschreibung.

Abholen is a separable verb.

Its base form is:

• abholen = to pick up

In a main clause, the prefix ab- separates and moves to the end:

• Ich hole die Krankschreibung ab.

That is why the sentence says:

• morgen hole ich die Krankschreibung ab

In a subordinate clause, the verb stays together:

• ..., weil ich die Krankschreibung abhole.

Because here they are adverbs, not nouns.

• heute = today
• morgen = tomorrow

Adverbs do not take articles.

Compare:

• morgen hole ich ... ab = tomorrow I pick ... up
• am Morgen = in the morning

So morgen is lowercase here because it is an adverb meaning tomorrow, not the noun Morgen meaning morning.

This is the usual difference between the indefinite article and the definite article.

• eine Krankmeldung = a sickness notification
• die Krankschreibung = the specific doctor’s note / sick-leave certificate being referred to

The speaker is introducing a notification, but refers to the certificate as something specific and identifiable in the situation.

English does the same kind of thing:

• I’m sending a message today, and tomorrow I’ll pick up the certificate.

Because they are nouns, and German capitalizes all nouns.

So in this sentence:

• Fieber = noun
• Krankmeldung = noun
• Krankschreibung = noun

That is why they begin with capital letters.

By contrast:

• wegen, heute, morgen, schicke, hole are not nouns, so they are not capitalized unless they begin a sentence.

Usually, no. In a sentence like this, the comma before und is often omitted in standard German:

• Wegen des Fiebers schicke ich heute eine Krankmeldung und morgen hole ich die Krankschreibung ab.

The version with the comma is understandable and may be used to mark a stronger pause or make the structure clearer, but many writers would leave it out.

So for learners, the safest takeaway is:

• before und, a comma is often not needed

Yes. German word order is flexible as long as the verb-second rule is respected in each main clause.

For example, you could also say:

• Ich schicke heute wegen des Fiebers eine Krankmeldung, und morgen hole ich die Krankschreibung ab.
• Heute schicke ich wegen des Fiebers eine Krankmeldung, und morgen hole ich die Krankschreibung ab.

The version in the original sentence puts Wegen des Fiebers first, which gives extra emphasis to the reason.