Don't shout bomb at airport security, don't scream fire in a packed theatre, don't mention the war in Germany and, for the love of all things holy, do not joke that you have Ebola on a plane that is about to take off.
That's exactly what one passenger did in the surreal video above, prompting scenes that wouldn't look amiss in ET.
Highlighting just how gripped the world is with Ebola hysteria, the plane passenger sparked panic after reportedly shouting "I have Ebola, you're all screwed" on a flight with 290 passengers.
He also yelled out, “I’ve been to Africa!” an airport official apparently told Fox News Latino.
When the US Airways flight 845 - travelling from Philadelphia, US, to Punta Cana, Dominican Republic - landed on the Caribbean island it was swiftly boarded by emergency personnel wearing blue hazmat suits.
In the video, a flight attendant can be heard exasperatedly saying on the plane's PA: ”I think the man who said this is an idiot, and I’ll say that straight out.”
Somewhat unsurprisingly, it turned out the unidentified comedian was not infected and the plane continued without him two hours later.
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Airport officials said the man will be returned to the United States, where he will "be submitted to another rigorous check."
The extreme reaction to the misguided joke comes as the government's U-turn on screening passengers for Ebola at UK airports threw operators into chaos on Friday morning.
One airport has already admitted it had no idea how to implement the measures as health experts said the project would come at vast expense and not help prevent the disease from spreading beyond west Africa.
The confusion came as it emerged a British man who was initially suspected to have died from Ebola in Macedonia was declared "unlikely" to have been suffering from the virus.
Experts have warned cases of Ebola in Britain were ‘bound to happen,’ but that screening would do little to identify infected passengers.
Checks will now take place at Heathrow, Gatwick and Eurostar terminals following advice from chief medical officer Professor Dame Sally Davies, amid growing criticism of the Government for delays in responding to the outbreak.
The World Health Organisation says Ebola has killed more than 3,800 people in West Africa. Liberia has been hardest hit with more than 2,000 deaths.