All species of flies have four stages of development within its life cycle which are the Egg, the Larvae, the Pupae and an Adult. Eggs of flies that migrate into a premises may never be seen due to being laid cryptically though species such as the Common/Lesser House Fly and Blowflies lay their eggs blatantly on foodstuffs, rotting materials and animal faeces which are minuscule, whitish in colour. After hatching from their eggs the larvae (maggots) may be seen in groups on their food sources and will undergo a series of moults over a duration of days or weeks (depending on the species) before Pupating then becoming an Adult. Maggots of other flies may not be seen due to opposed food sources and adults will vary in colour and size depending on their species.
Majority of female flies are capable of laying eggs within their 100's in batches or individually and their life expectancy can be anything from days, weeks, months or a year possibly giving them a long enough duration to thrive allowing the infestation to become severe. Giving their size and ability of flight, they are highly capable of gaining access to minuscule gaps of food packets, waste, household algae, pet faeces and more causing contamination and disease within a dwelling or working environment.