
Hi Jo,
No, I don't think you're silly at all (clearly you care, which is never silly!), but I think you're worrying unnecessarily. :) Who knows what wild animals go through their garden/yard at night? I know we have foxes, hedgehogs, mice and rats (I've seen them), but have raised litters of pups using the garden without any health problems.
Make sure your pup has it's jabs, and I'm sure you'll be fine. The greatest risk is from other (unvaccinated) dogs.
Good luck, and enjoy your pup! :)