SJC1K already answered for me. Smoking is something that affects everyone around, being fat affects primarily the person that's fat.

iff's mention of fat spreading through social networks seems to indicate the necessity of strangers to point out the obvious... Social checks inside the family network don't seem to work then.

I haven't heard about the link to viruses yet -- but it doesn't seem to be far-fetched. I just hope a cure is found (if it's true), otherwise it will be used as an excuse and lead to more weight gain.

HTB.