A lurker is a term used for the homeless living in Downbelow on Babylon 5.
Lurkers are mostly human although alien lurkers are not unheard of. They began settling on the station as soon as it was opened, beginning simply as people searching for new lives and opportunities. When they did not succeed, they often did not have the money to afford a ticket back home, so they ended up moving into the undeveloped parts of Downbelow.
There are those that who hide among the nameless in Downbelow because they are on the run from something or someone, like the Psi Corps or the law. Lurkers have hard lives, and as if being poor and homeless wasn't bad enough, they often become victims of the criminal underworld that calls Downbelow their turf. Often the only work for Lurkers is on the shady side. There have been situations where they are hired by smugglers, etc. to handle illegal cargo.
As these people search for items to trade, sell, and eat they are often subject to numerous hazards. From eating tainted, discarded food to being targets and victims.
Lurkers are seen in nearly every episode of Babylon 5. In most cases they are background characters receiving no credit or attention.