Death Cloud (Young Sherlock Holmes #1) by Andrew Lane

1868 England. Sherlock Holmes, age 14, unexpectedly summers with Farnham relations and befriends orphan Matt. Why does a dark cloud float from a corpse covered in red boils? Sherlock fights, runs, gets caught, drugged, whipped, ordered killed; defending bystander is fatally knifed. Americans logical tutor Amycus Crowe and daughter Virginia in well-fitting breeches help.