Like wasps and bees, ants are social insects and not usually involved in the spread of disease. They nest underground and are mainly found outside, but where nests are close to or under buildings ants will come inside. They are particularly attracted to sweet tasting foods so can cause contamination problems.
In the summer the male and female ants leave the nest in large numbers over a short period of time. Often this happens at all of the nests in an area. The male and female ants are larger that the worker ants, which we are used to seeing, and they are winged. If they get indoors in large numbers they can be quite frightening.