Some guests are polite and work with the host house to keep the house in order and running well(Mutualism). Some guest come in have a wild party reproduce like crazy and then break the walls down on the way out(HIV). There are many types of houses and many types of guest as indicated by the linked wiki article.
Feel free to be the first
Please review the linked page for context.
If you can think of something better than this,
please add it to the database