I have read various articles that litigating for damages due to injury against medical professionals or clinics is an uphill battle. See here
In simple terms, the case is as follows: A and B both got tested at different clinics for STIs. A got all negative. B was told they will be contacted in case something is wrong. B was never contacted. One year later A suffered from multiple injuries occurring due to an STI that A had. B got tested again and had the STI that A had (making B the source) at the same clinic. B was told by the clinic that the previous report (a year earlier) had "showed readings slightly more than average" but they did not inform B.
What are A's legal options in Germany?