There's a massive epidemic taking place across the US right now, though you probably haven't even heard about it. Over the last few months, more than 15 million chickens and turkeys have been infected with a deadly form of avian influenza, according to the latest numbers from the Department of Agriculture, and the country's bird disease experts can't figure out how it's spreading.

No one has been able to conclusively determine how the flu is continuing to spread and the effects have been dire. Dozens of countries have introduced bans or restrictions on importing US poultry, including China, South Korea, and Angola, where the US shipped a combined $700 million worth of poultry last year, according to Reuters.

There have been no reported cases of human infections, though the Centers for Disease Control and Prevention (CDC) has noted that there is a slim possibility the virus could make the leap. "While no human infections with these [viruses] have been reported worldwide, similar viruses (like Asian-origin H5N1, for example) have infected people in the past," the CDC website reads, though it makes clear that the risk is low.