  We give exponential upper bounds for $P(S \le k)$, in particular $P(S=0)$,
where $S$ is a sum of indicator random variables that are positively
associated. These bounds allow, in particular, a comparison with the
independent case. We give examples in which we compare with a famous
exponential inequality for sums of correlated indicators, the Janson
inequality. Here our bound sometimes proves to be superior to Janson's bound.
