Your parents warned you about them when you were younger. They hide in the dark crevices of your bed. They’ve been around for centuries. They feed on your blood. They move fast and only come out at night. Vampires? No — bedbugs.

Bedbugs are indiscriminate about their surroundings and can be a problem whether you live in a multimillion-dollar home or a single room occupancy hotel, according to Dr. Johnson Ojo of the San Francisco Department of Public Health. Your bed is the perfect breeding place. Here is a recently published bedbug registry.

Bedbugs produce 200 to 500 eggs in a lifespan of 12 to 18 months. The eggs hatch every 14 days, and San Francisco’s weather, especially the 70-degree temperatures common during the summer months, is ideal for breeding.

Though complaints seem to increase from year to year, Ojo said bedbug cases are underreported because most people don’t know they have them. San Francisco’s bedbug problems are not as bad as those of New York, Chicago and Houston. New York’s infestations most likely come from products imported from China and stored in warehouses.

What to look for? The reddish-brown creatures are a quarter of an inch long and are usually flat if not filled with blood. The bugs love humans — they are attracted to the heat and CO2 our bodies emit. They can live up to 18 months without blood.

Bedbugs are a public nuisance, but are not dangerous and are not known to spread diseases in humans. However, some people can develop allergic reactions to the bites.

“People can develop itchy welts that can lead to a secondary infection,” Ojo said.

When you scratch too hard and the skin breaks, bacteria enter the wound and cause infection.

Bedbugs have been with our civilization for centuries. Infestations were kept under control during the late 1930s and 1940s by the pesticide DDT, which was banned in 1972.

The problem with DDT was that “it killed everything in its path and was known to be a carcinogen,” Ojo said.

Though the United States eliminated bedbugs with DDT, the pests remained a big problem in areas of the Middle East, Asia and Africa. With the DDT ban and the increase in international travel, bedbugs were reintroduced in this country, and are now a problem in every state.

If you think you may have a bedbug infestation, don’t try to get rid of them yourself — you or your landlord should call a pest control company or the Department of Public Health. There is a protocol an investigator will follow to rid your home of bedbugs.

Look out for:

1. Bite marks on your body that itch.

2. Blood stains on the bedding.

3. Reddish-brown coloration in the seams of your mattress.

4. Musty smell in the mattress.