Quarantine transmitter
A quarantine transmitter was a device on some Starfleet starships which, when activated, would broadcast a general signal warning other vessels of hazards on board. This would be done upon suspicion of a dangerous disease or other hazard on board the vessel, and the transmitter could be activated remotely from another vessel. It would advise such vessels to not board the ship broadcasting the warning, at risk of picking up the hazard and passing it to other vessels. 

The  was equipped with a quarantine transmitter, and it would broadcast the following message:

"Extreme caution! The USS Lantree is a quarantined vessel by order of Starfleet Command. Do not board!"

The Lantree's transmitter was activated in 2365 by the  to warn vessels about an aging disease on the Lantree while they investigated it. After doing so, the Enterprise returned and destroyed the Lantree to prevent the spread of the disease. ()

Communications technology