Occasionally, a highly respected scientist will propose a wacky idea.  I remember in the late 1970s, Sir Fred Hoyle, a famous British astrophysicist, proposed the idea that many diseases were not passed by commonly understood ways, but arrived on Earth from space.

Hoyle claimed that the viruses and bacteria responsible for infectious diseases were really space invaders.  Diseases, from the common cold to killers like bubonic plague and smallpox, all he claimed originated when infected spores fell to Earth.  And he presented lots of statistical evidence on the spread of influenza in Britain to support his claim.

Because of his fame, his ideas got published, even though no one has ever observed a virus or bacterium in space, or one arriving from space.

Experts in the field of infectious diseases agree that transmission of contagious material involves spreading from one person to another.  Transmission can come through the air, as in the common cold, sometimes by personal contact, like AIDS, and sometimes through an intermediate such as a mosquito, as with malaria.  But all communicable diseases have one thing in common:  they originate somewhere on the surface of the Earth, and they are carried by terrestrial organisms.

So next time you read a wacky suggestion coming from a famous scientist, remember it’s evidence, not credentials that establishes the truth of an idea.