If when inspecting your hives you notice a strong smell, you should suspect American Foulbrood (AFB) the smell is said to be like old-fashioned hoof and horn glue.
AFB is the most serious bee disease. A virulent bacterial disease, it can be transmitted by hive tool, hands and anything which comes into contact with it.
Look for spotty brood with sunken, apparently oily, perforated cappings on cells containing dead brood, use the ‘matchstick test’ to check.
Take a matchstick and push it into one of the effected cells. If the dead brood can be drawn out of the cell with the stick in a ropey consistency and the material appears slightly elastic, snapping back into the cell like a rubber band, this is almost certainly AFB.
If you suspect you have bees with AFB, contact your state bee inspector in the US, official agricultural authorities elsewhere. They will check your diagnosis. If AFB is found, it is likely the bees and any associated equipment will have to be burned. Treatment of AFB is prescribed by state law in the United States.
AFB is very contagious and spores have been know to lie dormant for 70 years. No amount of cleaning or sterilization can be relied upon to kill the infection.
You should be extremely careful when buying secondhand equipment, especially if the owner’s bees have died out mysteriously. I know people who would not accept equipment from anyone, even as a gift, because of the danger of AFB.