[Phone]

I know the mechanism through which this spreads is apparent, but does anyone have any idea as to the root cause?

[Action, at and around 2238 Stevens]

A [Sarah Jane is standing in the doorway, watching a cluster of zombies and fiddling with her watch. She frowns, not liking what she's reading]

B [In a moment currently clear of zombies, she has ventured out to collect samples from the area immediately around the house in empty jam jars. She's wearing a set of men's clothing borrowed from a closet and heavy gardening gloves. Not taking any chances.]

C [Anyone who happens to actually find themself in the house will find Sarah Jane in the study, analyzing the samples with her regained equipment]