Just because we’re all hearing about H1N1 flu, and these terms are being used a lot, here’s the difference:

Infectious

1. A disease capable of being transmitted from person to person, with or without actual contact.
2. Syn: infective
3. Denoting a disease due to the action of a microorganism.

Contagious

Relating to contagion; communicable or transmissible by contact with the sick or their fresh secretions or excretions.
[from Stedman’s online Medical Dictionary]

Anthrax, for example, is infectious but not contagious. It is caused by a microorganism, the bacterium Bacillus anthracis, but it’s not “communicable or transmissible by contact with the sick or their fresh secretions or excretions”. People most often get anthrax from contact with infected hides or other animal products, and from soil where the hardy spores of the bacterium can remain for decades after being deposited by infected animals. [Such spore formation is known in only a few bacteria.]

There’s some confusion inherent in these terms because it seems that the “contact with the sick or their fresh secretions or excretions” part only applies to sick humans. You might get rabies from breathing in droplets of the saliva of an infected animal, but that is not considered to be contagion. As near as I can tell, anyway. So, since a human being with rabies doesn’t infect others, the disease is considered non-contagious.

An important factor in any contagious disease is how easily it is transmitted from one person to another. You can’t get HIV from touching the skin of an infected person, but influenza and the common cold can be transmitted that way. Shake hands with someone who just sneezed into his or her hand, and the bacteria are on your hand; when you touch your mouth, nose or eyes, the microorganisms can enter your system. TB is contagious, as is leprosy, but they are not transmitted by brief casual contact.

Right now the question about H1N1 flu is, how contagious is it? And then, how fatal is it? Influenzas mutate rapidly so the virus which seems to have originated in Mexico may be changing to something different as it spreads. Hence the reluctance of medical officials to make predictions about what is in store for the world with this disease.