I have been researching on diseases/illnesses that cause eye bleeding and i came across ebola, i researched a bit more on ebola and here are some symptoms that coincides with Patricks death (Fever and external bleeding) and cast and crew have hinted that diseases are the new threat.
Also on a second note i saw this walker in the episode
Shall we agree on the threat and cause of Patricks death is ebola?
Notice the bloody and red eyes