It is a very old custom for Hindus to take bath after visiting the house of a dead person, cremation ground and after seeing a dead body.
There is a lot of negative energy in the house of the dead person. The sadness, crying and wailing attracts negative forces. The negative forces are always on the lookout for a new body to make their home. They get attracted to our aura and try to enter it. This is one reason why Hindus take bath after visiting the house of the dead person.
Some Hindu communities put salt in water and take bath after seeing a dead body.
After death 10 pranas leave the human body. Some them only leave after fire catches the body. Some of these still linger around the dead body.
When visiting the dead person we come in contact with this pranas. It is also to avoid any negative influence from these pranas that Hindus take bath after visiting the cremation ground.
It is also believed that after life force has exited from the human body, the body becomes home to numerous parasites and other small living beings. Some these are disease carrying. It is to wash away this too that bath is taken after sighting a dead body.