Walking Dead
Rick came across a sick boar when he met Clara in the woods during the season 4 premiere of "The Walking Dead." Gene Page/AMC

WARNING: MAJOR SPOILERS AHEAD. Do NOT continue reading if you have not watched the season premiere of “The Walking Dead.”

“The Walking Dead” returned for its fourth season on Sunday, Oct. 13, and showed no signs of letting the prison survivors off easy. Despite new hope brought on by safer and more civilized living conditions, the group has a new threat on their hands.

“It’s not even the walkers, it’s not even other people … it’s a whole new threat,” Maggie’s Lauren Cohan teased E! Online of the problems that the group will encounter this season. And according to series creator Robert Kirkman, this new threat “could be more deadly than the other two combined.”

In season 3, Rick’s group had to “fear the living.” Thanks to clues sprinkled throughout the premiere, “30 Days Without An Accident,” it now it appears as if the survivors have to fear an invisible virus.

During episode 1 of season 4 there were a few clues on the new threat that fans could have found if they watched closely:

1. Sick animals -- One of farmer Rick’s pigs appeared to be sick at the beginning of the episode. When he ventured into the woods to search his traps he discovered a sick boar (and crazy Clara) lying on the ground. Suspicions on these ill animals grew when Rick returned to the prison to find his pig dead.

2. Bleeding Eyes -- A lot of walkers appeared to be hoarding around the fence, but the camera kept focusing in on one zombie. Since it's been a couple years since the zombie apocalypse began, a majority of the walkers are pretty decayed. However the zombie in question appeared to look pretty fresh ... and obviously bleeding from its eyes before turning.

3. Poor Patrick -- Patrick, a new young boy in the prison, started off the episode well. However, mid-episode he began to not feel well, telling Carol that he felt like he was going to throw up. By the end Patrick appeared to have a fever and nausea, and the premiere concluded with him collapsing in the showers … and reawakening as a zombie with bleeding eyes.

“It is safe to say Patrick died of what ever that was,” showrunner Scott Gimple revealed to the Hollywood Reporter. “The walker that was at the fence that we saw was rather fresh; he’s not very rotted. I would intuit that because it’s fresh zombie, that the person who died recently was by the same method Patrick died.”

But how did Patrick die? Our theory is Ebola.

According to the National Library of Medicine, Ebola hemorrhagic fever (Ebola fever) is a human disease that can be passed from infected animals. However, it has mainly been limited to Africa, and the Center for Disease Control reports that “no case of the disease in humans has ever been reported in the United States.”

How the virus first appeared in humans has not been determined, but it’s speculated by researchers that the first patient became infected by contact with an infected animal. Once one patient becomes infected it is easy for the virus to be transmitted. Exposure can come via a health care setting that has no means of protection (like masks, gloves and gowns), direct contact with blood or secretions or contact with objects like needles.

The CDC reports that symptoms of Ebola fever include:

-Fever

-Headache

-Joint and muscle aches

-Sore throat

-Weakness

-Diarrhea

-Vomiting and stomach pain

- And in some cases rash, red eyes, hiccups and internal and external bleeding.

“Walking Dead” viewers will remember that Patrick complained of stomach pains and the need to vomit while he was in the library with Carol. When we saw him again in the bathroom he appeared to be suffering from a fever, trying to cool himself down with a shower. After collapsing, the cameras showed him covered in blood, appearing to bleeding from his eyes.

What do you think happened to Patrick? Let us know your thoughts on the “new threat” to hit the prison in the comments section.

New episodes of “The Walking Dead” air on AMC on Sundays at 9 p.m.