A miasma is a cloud of foul-smelling vapor, like swamp gas. You could have a miasma of sweat that lingers in a locker room long after a soccer team has left, or a miasma of rumor swirling around a politician.
As you've probably figured out, miasma is always negative. You could describe the miasma of a hot summer day, but you'd be thinking of the stench of uncollected garbage and exhaust fumes, not the smell of summer flowers. From the Middle Ages until the late 19th century, when germs were identified as causing disease, people believed that you could get sick from the miasma of everyday life — from the odors arising from poor sanitation and cramped quarters.