Used Military boots in Grade 1, re-issue condition are pretty safe. I used to be a Combat Medic, and foot infections were rare due to regular foot inspections. If you're really worried about the risk of fungal infection, the best advice I can give is to spray the inside of the boot with a clinical disinfectant, let it dry and then apply a liberal quantity of anti-fungal foot powder to the inside and leave it for a couple of days before you wear them. I do this to every pair of used boots I get and never had any problems.
No man knows true happiness until he's married......but then it's too late!
Proud member of the Greater Manchester Bushcraft Group