If you are seeing damage to the hosta you are either having top feeding which is NOT from the grubs or nematodes. Grubs nibble at the roots and the damage is not visible. Nematodes are inside the hosta and micoscopic, their damage will not be visible until July. If you are having damage to the roots it might be voles though.
The nematodes in the hosta will not infect humans because the environment is so different. Ph, cell stucture, temperature etc. Plants have totally different cellular structure. They can infect other plants though.