A contagious illness, or a pathogen causing it.

• He's got the flu bug.

informalAn enthusiasm for something; an obsession.

• I caught the skiing bug while staying in the Alps.
• As we rode in the bus in the weird phosphorescent void of the Lincoln Tunnel we leaned on each other with fingers waving and yelled and talked excitedly, and I was beginning to get the bug like Dean.

informalA keen enthusiast or hobbyist.

• His mother had been a bug on astrology, which was why the moment of his birth had been impressed on him so exactly.
• Incidentally, the camera manufacturers have had a new worry—that they might "kill off the hobby," as U.S. Camera magazine put it recently—by automating to the point that real camera bugs would feel no challenge.

A concealed electronic eavesdropping or intercept device

• We installed a bug in her telephone.

A small and usually invisible file (traditionally a single-pixel image) on a World Wide Web page, primarily used to track users.

• He suspected the image was a Web bug used for determining who was visiting the site.

broadcastingmediatelevisionA small, usually transparent or translucent image placed in a corner of a television program to identify the broadcasting network or cable channel.

• Channel 4's bug distracted Jim from his favorite show.
• The score bug displays the current football score over the ongoing match.

aeronauticsaerospaceaviationA manually positioned marker in flight instruments.

• You look up the proper speed for the phase of flight, set the reminder bug, and then literally forget the speed. You don't read the airspeed number, you fly to the bug.

A semi-automated telegraph key.

• At this point your telegraph operator, sitting at your right, goes "Ticky-tick-tickety-de-tick-tick," with his bug, as he calls his transmitter, and looks at you expectantly.
• As far as the dashes are concerned, the bug is the same in operation as any regular key would be if it were turned up on edge instead of sitting flat on the desk.

obsoleteHobgoblin, scarecrow; anything that terrifies.

• Sir, spare your threats: / The bug which you would fright me with I seek.
• In old wives tales that in old time did live, To whose odde tales much credit men did give, Great store of goblins, fairies, bugs, nightmares, Urchins, and elves, to many a house repaires.

HIV.

• The arguably most debated bareback practice that came to attract attention early on (and still does) was that of "bug chasing," in which HIV-negative men (bug chasers) actively seek out sex with HIV-positive men (gift givers).

card-gamespokerA limited form of wild card in some variants of poker.

datedslangSynonym of oil bug.

• Now, only three years later, most of the major oil companies maintain staffs of these men who examine cores, classify the various types of "bugs," or foraminifera, and make charts showing the depths at which each of the hundreds of types is found.

slanghobbieshorse-racinghorseracingAn asterisk denoting an apprentice jockey's weight allowance.

• The "bugs" are the asterisks next to the apprentice's name. One bug is a five-pound allowance, two bugs equal seven pounds, and three bugs equal ten pounds.

slanghobbieshorse-racinghorseracingA young apprentice jockey.

mediaprintingpublishingSynonym of union bug.

slanggamblinggamesA small piece of metal used in a slot machine to block certain winning combinations.

• Because many illegal slot-machine operators here and abroad do not like to give the slot-machine player even one chance to hit the jackpot or the big bonus, they make use of a "bug." This is a small, flat half-circle of iron about an inch long, which looks something like a bug.

slanggamblinggamesA metal clip attached to the underside of a table, etc. to hold hidden cards, as a form of cheating.

• Some clumsy or audacious sharpers will go so far as to hold out cards in their lap, or stick them in a "bug" under the table.
• Fargo had been in a saloon in Kansas when a man was caught using a bug. Made of steel and shaped like a money clip with two sharp ends, the bug was jammed under a table and held cards the bug's owner palmed until they were needed.