Particularly bad depressive episodes are similar to the flu/mono/etc.: you are incapacitated, feel like shit, and can't get anything done. Yes, they can flare up without warning: so can the flu. Yes, they can last a long time (DSM diagnosis requires more than 2 weeks): so can something like mono or cancer, which get sympathy, not fear.
Stigma is almost entirely a cultural thing: I know of someone who's first manifestation of mania (bipolar disorder, type 1) was in a religious setting... so when she said she heard God talking to her, they were reportedly thrilled. It didn't come out that it was a problem until she was on the plane home and was blessing the passengers (edit: details slightly obscured/changed).
Stigma is almost entirely a cultural thing: I know of someone who's first manifestation of mania (bipolar disorder, type 1) was in a religious setting... so when she said she heard God talking to her, they were reportedly thrilled. It didn't come out that it was a problem until she was on the plane home and was blessing the passengers (edit: details slightly obscured/changed).