Assuming your tables are MyISAM (the default) then you can look into using fulltext indexing and searching (aka, match(field1,field2,field3) against('word")... ). You can grab results using a single query if all th fields are in one table, in they're in multiple tables you can do a union of the results.
To do this you'll need to add fulltext indexes to each of the fields you want to search. For performance sake, the default settings create indexes for words greater than 3 characters, but you can change that setting if need be.
Here's a helpful page on fulltext searching: