How and why does Liesel's brother die?

Werner's illness is not specified, but he may have had TB or Typhus.

From the text:

The book thief and her brother were traveling down toward Munich, where they would soon be given over to foster parents. We now know, of course, that the boy didn’t make it.

HOW IT HAPPENED There was an intense spurt of coughing. Almost an inspired spurt. And soon after—nothing.

When the coughing stopped, there was nothing but the nothingness of life moving on with a shuffle, or a near- silent twitch. A suddenness found its way onto his lips then, which were a corroded brown color and peeling, like old paint. In desperate need of redoing.

He was very sick. His condition wasn't specified in the book. He might've had Typhus or TB. Sad that he died.