Lightwood's law is the principle that, in medicine, bacterial infections will tend to localise while viral infections will tend to spread.[1] This is based on the observation that while bacterial sepsis tends, despite affecting the whole body, to have a clear site of origin or 'focus', the opposite may be true of viral infections.[2] There may be multiple sites across the body which are affected including dermatological manifestations, respiratory symptoms and gastrointestinal symptoms.
This principle is by no means infallible and in clinical practice a variety of diagnostic tests are used to distinguish between bacterial and viral infections.