While no monsters are ever actually seen in "real life" sequences, all of the other aspects are definitely there, and the infected grandfather's appearance and behavior in his grandson's nightmares are very suggestive.
While it's possible that the dreams are triggered by a fear of the plague itself and burning the bodies of the dead is likely safer than burying them, this doesn't explain what happened to the dog. It was very clearly attacked by something that also seems to have been intelligent enough to bring its body back to the house. The drawings of figures in the forest also seem to suggest a conscious threat, rather than just paranoia mixed with the threat of disease, at least when mixed with this scene.
An airborne zombie plague that results in violent but intelligent victims would have an incredibly bleak outcome. There's no "fighting back" in that situation. Just hiding in the woods, in some remote locality, and waiting to die one way or the other. Actually seeing the monsters close up would mean death, either at their hands or by infection. The result is a claustrophobic horror film with little explicit action or terror.
