What would cause fluid to leak into the spark plugs of a 4 cylinder 2002 Nissan Altima?

If it is coolant that is leaking, it may be due to a blown head gasket or warped/cracked cylinder head. If it is fuel that is leaking, the spark plug or wires are faulty and there is not enough spark to fully ignite the fuel. If it is oil that's leaking, the valve seals or piston rings may be worn.