"Uh... Hi Harold..."
You stammer for a second in an attempt to stall for time. Finally an brilliant thought pops into your head.
"I've been meaning to call you for quite some time! I'm sorry I haven't answed any of your questionnaires yet, but apparently my computer contracted some AWFUL virus that lodges itself into anything I write and multiplies within documents, so anything I send people is saturated with little bugs. I didn't think you'd want to risk infection so I've held off on responding to any of your emails. I hope it's not too much of a problem."
Harold scrutinizes you and wipes his mouth on his sleeve. He gives a throaty cough and a nasty sneeze, remeniscent of those on the mysterious message, and begins to tell you off.