How are drain flies a health hazard?

As indicated in an article with the Singapore Science Centre:-ScienceNet - Life Sciences - Safety/ Health: Article #: 9602

Flies belong to the class of Insecta and make up the order Diptera. Moth flies (or drain flies) make up the Family Psychodidae. Drain flies are small, dark, fuzzy, moth-like insects. Drain Flies are called moth flies because of the long hairs on their wings, perhaps they look like small moths. These flies do not move far from the breeding site. They don’t fly well at all and usually spend their time close to the site they are infesting. They are often found clinging to the walls of bathrooms, kitchens, or in the basement. During the day, adult flies rest on walls or on the sides of tubs and shower stalls. They become more active at night, hovering over drains, sinks and other breeding areas. As their name implies they are associated with clogged drains, but also to organic matter that may accumulate around sinks and other fixtures in bathrooms. The larvae can survive some fairly strong chemical applied to the drains, so the best method for control is to physically clean (remove the stuff) the drain. These flies rarely occur in large numbers, but may persist throughout the year in small numbers.

Drain flies breed in polluted shallow water or the scum that often collects around drains. In the house, drain flies lay their eggs in the gelatinous material which accumulates on the sides of drains and overflow pipes, and the larvae feed on the decaying organic matter and microscopic plants and animals that occur there. Each female can produce about 100 eggs and, under optimum conditions, development can be completed in two weeks.

Although these flies do not bite, they can be annoying in the house. Control of drain flies should be aimed at control of the breeding sites. The most effective control method is to clean the drain pipes with a stiff brush, removing all the slime in which the flies breed. Sometimes it may be necessary to remove the trap to thoroughly eliminate the breeding media. Pouring hot water down the drain provides short-term control. Drain fly larvae are difficult to drown because they are able to trap air bubbles and remain submerged for a day or more.

To my evaluation & interpretation of the excerpt. It appearers as if this particular type of fly holds so much of a health risk, as it does a nuisance.