According to this document, these were replacement personnel intended to keep numbers up after casualties were incurred:
"The automatic replacement drafts, devised by the War Department, were intended to furnish each month the equivalent of the estimated casualties for the following month. But the AEF was still drawing men from training divisions and from the Services of Supply to obtain the replacements needed for its combat units. There was a constant demand for as many men as could be sent overseas, irrespective of their state of training. The men who were called under the automatic replacement draft during July had received scarcely 2 weeks of actual training before their departure for overseas; and similar conditions continued during August and September."
It seems highly likely that young Lem may have been sent over with a serious lack in training.
How do you know he died of disease? There's nothing in the document to show this as a fact.