Worm infestation is a very common infection. It is caused by different kinds of worms that can affect humans, such as tapeworms, round worms, pin worm. These worms are parasitic in nature therefore they derive nutrients from their hosts.
Worm infestation can be contracted very easily unlike many other infections, hence the need for periodic treatment (deworming) together with improvement of water, sanitation, and health education.
This infection can be contracted through any of these means:

School going children in your family can get you the eggs/larvae of these parasites
Walking about on barefoot
Eating under cooked meat of infected animals

According to the World Health Organisation (WHO)

WHO recommends periodic treatment with anthelminthic (deworming) medicines, without previous individual diagnosis to all at-risk people living in endemic areas.

People at risk are:

preschool-aged children
school-aged children
women of childbearing age (including pregnant women in the second and third trimesters and breastfeeding women)

Treatment should be given once a year when the prevalence of soil-transmitted helminth infections in the community is over 20%,

and twice a year when the prevalence of soil-transmitted helminth infections in the community exceeds 50%. This intervention reduces morbidity by reducing the worm burden.

source: http://www.prisschar.com/2016/06/how-often-should-you-deworm-yourself_28.html