Occasionally, Sherlock did require specialist knowledge. Some easier to come by than others, like this case for instance, altogether it was not that smart, the case was closed and the killer was caught but it had shown a weakness to Sherlock that under normal circumstances would not have occurred to him.
So, here he was, exchanging emails with an army captain who happened to also be a doctor currently stationed in Afghanistan about knife rituals and IED injuries. It started rather boring. Short messages, straight to the point and no useless chatter but the longer this back and forward continued the more they spoke. 3 Months later they spoke on IM as regularly as possible.
It was a surprise to Sherlock, Captain Watson, he really should be rather boring but Sherlock found him intriguing. What was more of a surprise was that Watson (Cpt. John Hamish Watson) wasn't scared of him and didn't think of him as a freak, but found him fascinating (his words). So, when available, Sherlock had his own Dr. to speak with about medical knowledge on his cases.
It was another 3 months later that they arranged to meet when John had leave at the weekend. Within a few hours of meeting, they both sat in chairs in Baker Street drinking tea and talking, John still in his uniform when Sherlock's phone started ringing, it also came as a surprise to Sherlock that he denied the DI's call and continued talking with John. Half an hour later the door went and Lestrade came up with a case file, Sherlock scoffed at it and handed it to John with a raised eyebrow. Sherlock surprised them all by laughing when John looked at the file and frowned disbelievingly at Sherlock before handing the file back to the DI and saying "It was the nephew" before continuing to drink his tea.