If you've ever dealt with head lice, you'll know how difficult it is to get rid of them.

There's a range of products in pharmacies — from special combs to shampoos — but sometimes it's not enough.

You need to understand how they live, spread and survive to beat them to get rid of them once and for all.

What are head lice?

The US Centers for Disease Control and Prevention describes head lice as insects the size of a sesame seed.

Lice feed on human scalp blood. When they suck blood, they release a spit that can cause an allergic reaction and an itchy sensation. They live for about 30 days.

Female lice can lay up to eight eggs per day. They crawl — they cannot jump and do not fly.

Unlike body lice, head lice are not dangerous and do not carry diseases.

How do lice spread?

Lice spread through direct contact — they crawl from one head to another.

They can also spread via scarves, combs, towels, hair ties, pillows, sheets and coats — almost anything that's had contact with an infested person's hair.

"If they fall off a head, they can survive for 48 hours on a surface without feeding. As soon as someone enters the room, they will move toward him or her to feed," said Rosario Melero, an expert in medical parasitology at the Fundación iO in Madrid, Spain.

Can lice survive in water?

Yes, lice can survive for several hours in water. In fact, you can even get them at swimming pools.

"Their body allows them to float," Melero said, adding that they "find it much easier to cling to a head" when hair is wet.

Who is most at risk of getting lice?

Children are most at risk of getting lice.

"[Kids] don't have hormonal changes," said Melero. And lice like that.

They also have much thinner skin than adults, which makes it easier for a louse to bite.

"Maximum efficiency, minimum effort," said Melero. "But it also helps that [kids] have much closer contact with each other [than adults], putting their heads together at school or while playing."

Yael Adler, a dermatologist in Berlin, said girls, who often have longer hair than boys, were more likely to get them. "Lice have it easier, moving from one strand to the other [when hair] is long," Adler said.

Similarly, in families it is often the mothers who get lice from their kids.

"[Mothers] are usually the ones who delouse their children and often have long hair," Melero said.

But that can be true for anyone — of any gender — with long hair, who helps children with lice. 

Does it help to wash your hair more regularly?

No. Melero said clean hair makes it easier for lice to reach the scalp.

Why are lice so hard to get rid of?

One of the main problems is identifying lice and their eggs in the first place — if you can't find any, you might think you're okay.

"There may be only one louse on the scalp that you can't detect. Especially on dark hair. If it lays eggs, it will lay them behind the ears and they are so small that they can be mistaken for dandruff," said Adler.

Melero said it would help if there were more of a culture of prevention with lice, as there is with mosquitoes and malaria.

"There are repellents that you can apply to the head to prevent lice from biting or laying eggs. It is also important to use a fine-toothed comb several days after the last time lice were seen on the head to eliminate any hidden eggs," Melero said.

What can you do if you have lice?

The most common treatment is insecticidal shampoos. They need to be applied several times to kill all the living lice. But they are less effective against lice eggs, which are called nits. You can apply a silicone oil, soaking the hair and scalp.

"After 10 minutes, all the lice will be dead and you can remove them with a nit comb. The whole process should be repeated after eight days in case there are any nits left. It is also recommended that the rest of the family receive the same treatment to avoid any transmission," Adler said.

Wash clothes that have been in contact with lice at 60 degrees Celsius (140 F).

Put any items that can't be washed at high temperatures in a freezer for several hours, or seal them in a bag for a couple of days.

And always consult your family doctor or general practitioner.

Edited by: Zulfikar Abbany

Sources:

Parasites A-Z Index, US Centers for Disease Control and Prevention https://www.cdc.gov/dpdx/az.html

Veterinary Parasitology, North Carolina State University https://parasitology.cvm.ncsu.edu/