Yes, it's true that dogs have been known to detect certain diseases and medical conditions in humans. Dogs have an incredibly keen sense of smell, and they can detect subtle changes in odors that are associated with various health conditions. Here are a few examples:

1. Cancer Detection: There have been reports of dogs being able to detect certain types of cancer in humans, such as melanoma, lung, breast, and ovarian cancer. Researchers believe that dogs can sense the volatile organic compounds released by cancer cells.
2. Diabetes Alert Dogs: Some dogs are trained to detect changes in blood sugar levels in people with diabetes. They can alert their owners when their blood sugar is too high or too low, allowing them to take appropriate action.
3. Seizure Alert Dogs: Certain dogs are trained to sense an oncoming seizure in individuals with epilepsy. They can provide an early warning, allowing the person to get to a safe place or take medication if needed.
4. Narcolepsy Detection: Dogs have been trained to recognize the signs of narcolepsy in their owners and alert them when they're about to experience a sudden episode of sleep.
5. Migraine Alert Dogs: Some people with migraines have reported that their dogs seem to sense when a migraine is coming on and provide comfort.

While these anecdotal reports are fascinating, it's important to note that the scientific research in this area is ongoing, and not all claims have been fully substantiated. Training dogs to consistently and reliably detect specific diseases is a complex process, and the results can vary from one individual dog to another.

However, there's growing interest in the potential of utilizing dogs' olfactory abilities in medical settings, and researchers are exploring ways to train dogs to detect diseases through their sense of smell.