The celebrities, who often speak up about animal-rights, made the comment in an Op-Ed for major publication The Washington Post.
'Proliferation of superbugs'
"Less notorious [than wet markets] but much more commonplace threats to public health are the 'concentrated animal feeding operations' (CAFOs) scattered throughout the South and Midwest," Phoenix and Mara wrote.
"These factory farms warehouse thousands of animals that wallow in their own waste with limited or no airspace, routinely creating conditions for the proliferation of superbugs and zoonotic pathogens."
The piece added that 'nearly the entire supply' of animal products consumed in the US come from these types of factory farms.
This isn't the first time factory farming has been slammed for its risk of spreading disease. The BBC warned intensive farming 'must change' to avoid future pandemics in a recent documentary.
Veterinary pathologist Professor Andrew Cunningham, who specializes in wildlife diseases, said: "It's not a coincidence that we talk bout avian influenza and swine influenza, those are the two species that are farmed intensively the most around the world.
"Thousands and thousands of animals packed in together and that's a very good environment for a new virus to emerge from a mutation."