Former Vice President Joe Biden is defending his record on Social Security, amid increased criticism from the campaign of Vermont Sen. Bernie Sanders, which has repeatedly pointed to Biden's willingness as a U.S. senator to back spending freezes for Social Security and other entitlement programs.

The candidates' back-and-forth culminated — as most disputes in modern presidential campaigns do — in dueling Twitter posts Tuesday night.

A new strain of coronavirus — named 2019-nCoV — has been discovered in China. Viruses in the coronavirus category can cause fever, breathing difficulties, pneumonia and diarrhea. Some are potentially fatal. Others can cause the relatively mild common cold.

Here is a look at the most well-known coronaviruses: what they have in common and how they differ.

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More than three months after they began, protests in Iraq have escalated and taken a new turn this week. Anti-government demonstrators are attempting to force drastic change in a country whose government is in turmoil and grappling with a crisis between Iran and the United States.

Forecasters in southern Florida had warned a sharp cold snap could bring a high chance of falling iguanas – and that's just what happened: The National Weather Service's office in Miami says immobilized iguanas began falling from the trees after temperatures plunged into the 30s and 40s early Wednesday.

It was Day Two of the House managers' opening arguments in another Senate impeachment trial 21 years ago. Bob Barr, a Georgia House Republican,
was speaking in the well of the Senate, making a case for removing President Bill Clinton from office.

"We urge you, the distinguished jurors in this case," Barr said to the assembled senators, "not to be fooled."

Memphis Meats, a Berkeley, Calif.-based startup, says it's one step closer to bringing cell-based meat to consumers' mouths.

The company plans to build a pilot production facility with funds raised from high-profile investors including Bill Gates, Richard Branson and Kimbal Musk, as well as two giant players in the animal protein and feed space, Cargill and Tyson Foods. The company says its latest funding round has brought in $161 million in new investment.

When hundreds of migrants from Central America waded across a river from Guatemala into Mexico on Monday, chaos broke out. For Saury Vallecilla Ortega, a single mother of four, a nightmare ensued.

For more than a day, she was separated from her 5-year-old daughter Andrea and feared for the worst.

Chinese state media are reporting that authorities in the city of Wuhan are planning to suspend bus, subway, ferry and long-distance passenger transport. They also say that all flights and trains departing from Wuhan will be temporarily canceled in a bid to reduce spread of the new virus.

The announcement from Wuhan authorities read in part: "Residents should not leave the city, unless under special circumstances. Outbound planes and trains will be halted temporarily until further notice. We appreciate your understanding and support."

The District of Columbia is suing President Trump's inaugural committee, the Trump Organization and the Trump International Hotel in Washington, accusing them of "grossly overpaying" for event space at the hotel to enrich the president's family during the 2017 inauguration.