Learn Swedish: Penicillinet var ett otroligt genombrott, vilket botade sjukdomen. - Penicillin was an incredible breakthrough, which cured the disease.

This is the categorical use of the definite singular. In Swedish, when you talk about an invention, substance, or phenomenon as a general concept, you often use the definite form. While English drops the article for Penicillin, Swedish uses Penicillinet to represent the substance as a whole class.

Could I use som instead of vilket for which in this sentence?

Yes, som is perfectly fine and more common in everyday speech. However, vilket is often used in written Swedish to refer back to a neuter noun (ett genombrott) or an entire preceding clause, especially after a comma when adding extra, non-essential information.

How is the word genombrott constructed?

It is a compound combining genom (through) and brott (a break, or a crime). This forms a direct word-for-word translation, known as a calque, of the English word breakthrough.

Is sjukdomen also an example of the categorical definite form in this sentence?

Not in this case. Here, sjukdomen literally means the disease, implying a specific illness mentioned earlier in the broader context. If you wanted to talk about curing disease as a general, categorical concept, Swedish would usually use the plural form sjukdomar.