The original story…

“Timeo Danaos et dona ferentes” means “I fear the Greeks, even when they bear gifts.” It’s led to “Beware the Greeks bearing gifts.” These are all allusions to the Trojan Horse, which managed to secure a decisive victory for the Greeks during their ten year long war against the Trojans.

The Trojan horse, a gigantic hollow wooden horse, was the idea of Odysseus (aided by his patroness Athena), as a means of entering the impenetrable walls of Troy. Some of the Greeks hid themselves within the horse while others sailed their ships towards a temporary location, leaving the Greek encampment empty but for the horse. To the Trojans, it seemed as if the Greeks had finally given up after ten long years. They examined the horse and found one Greek, Sinon, still remaining. Sinon explained that the Greeks had left the horse as a sacrifice to the gods. He added that if the Trojans were to bring the horse within the city, it would bring the favor of the gods to the Trojans. Only one man, Laocoon, saw through the ruse, but Poseidon sent sea serpents to strangle Laocoon and his two sons. To the Trojans, it seemed as if the gods were punishing Laocoon for his lies.

The Trojans brought the horse into the city, even breaking down part of the wall because the horse would not fit within the gates. That night, while the Trojans slept, the Greek warriors within the horse crept out and ransacked the city. From the inside, they were able to let in the remaining Greeks. Thus the Trojan War was won.

The modern usage…

Today, “Trojan horses” are used in the context of computer software. A trojan horse is now malicious software (malware) that masquerades as benign software. Like the original Trojan Horse, the software is downloaded and installed. Perhaps it’s been advertised as a virus-detection program. Once installed and run, the virus then proceeds to act maliciously, perhaps deleting everything on the computer.