Relf was 33 when he died from electrocution, in the basement of his home, while playing his improperly earthed (electrically grounded) guitar. Relf had dealt with several health issues throughout his life, including emphysema and asthma, which may have contributed to his inability to survive the electric shock.
If you have ever heard the Yardbirds version of (H. Wolf's) "Smokestack Lightning" Kieth did the harmonica solo. Maybe the best harmonic solo ever.
Thanks Mr. Rief