2 billion people have worms. That's what the World Health Organization says. 

These are the kind of worms that spread when sanitation isn't enough. Not everyone has a toilet. There are about 7 billion people in the world; 4.5 billion have working toilets. That means more have cellphones than toilets. Without the luxury of a toilet, some use the outdoors as their toilet. 2.5 billion don't have proper sanitation.

Over 1 billion practice open defecation.

When not everyone has a toilet, everyone can be at risk for worms. Worm eggs get into the soil when there aren't enough toilets and people find other places to relieve themselves. Many of these worms are acquired simply by walking barefoot (strongyloides, Hookworm) - in areas where human waste has contaminated the soil.

Other worms spread when there aren't food safety regulations aren't enforced. Other worms spread when people can't wash their hands as much as they should.. Infections can be ingested as eggs pass from unwashed hands to mouths,such as with pinworms that spread from child-to-child, such as when children share a toy contaminated by dirty hands.

This means that:

Ascaris infects 800 million to 1 billion people. It spreads when human waste becomes fertilizer; eggs in the soil can end up as dirt on fruits and vegetables that are eaten. Most infections aren't recognized, but the infection can stunt growth in children and even cause intestinal blockage.

These worms can be several centimeters long - even up to 30-40 centimeters.

Whipworm (Trichuriasis, Trichuris Infection) infects up 1 billion people, and at least 600 to 800 million. This infection is more common in the South of the US than many think. Many don't notice the infection. Children may have slowed growth.

Others may have anemia or have difficulty passing stool as the worms living in the intestines can cause blockages. This can even lead to rectal prolapse.

Hookworm infects 570-740 million. The Hookworm was once common in the South of the US. These worms enter through skin (especially the feet) and wander up through the lungs and down to the intestines where they attach themselves and can cause anemia (by eating up our blood). Shoes and good sanitation matter.

Pinworms infects 200 million. This is a common worm in small children everywhere. These are the most common worms in the US. The CDC reports that among children, child caregivers, and those who are institutionalized rates can reach up to 50% at times. These worms can stubbornly remain on toys for 2-3 weeks, leading to recurrent infections at preschools, as toys are reinfected by kids who scratch their bum or don't wash properly after using the toilet. They spread anus to mouth.These are tiny worms that can be visible to the naked eye.

Placing and then pulling a piece of tape can catch the worms.

Strongyloides infects 30-100 million people. Rates are low in the US, but can be very high among immigrants, reaching up to 10%-40% and even 60% in some countries described as tropical or subtropical. The infection can be chronic and unnoticed, until someone develops an immune deficiency - such as from HIV, HTLV, or from having cancer treatment or immunosuppression for arthritis. Infections in the immunocompromised can be very serious.

Taenia infects 100 million people. Half infect through beef (Taenia saginata) and half through pork (Taenia solium and asiatica). Taenia solium is a major cause of epilepsy worldwide.It causes cysts in the brain, called neurocystercicosis, which can cause high rates of epilepsy where pigs wander. Once neurocystecicosis occurs the infection cannot simply be treated away, like many intestinal parasite infections can be. It can also be transmitted by dirty hands from a food preparer who is infected; even those who do not eat pork can be infected through food preparation.

Toxocara infects more than we thought. In the US, there are likely hundreds of thousands of people, maybe even over a million, who have positive antibodies to Toxocara, meaning they have been exposed and/or infected. Toxocara canis and Toxocara cati are the most common worms in dogs and cats, respectively. The worms are found in pet intestines and end up in dog poo and kitty litter. However, you don't need to pick up dog poo to be affected. Petting an animal that hasn't been dewormed and then eating can lead to infection. Many will not have symptoms; some will. These infections include the Visceral Larva Migrans (which can be serious), Ocular Larva Migrans (which can affect vision and cause eye pain), and Cutaneous Larva Migrans (where worms can wander under the skin).

Trichinellosis is acquired by eating meat with trichinella cysts. It's a disease that is increasingly related to wild game hunting. This used to be from pork, but now it's often from wild game (wild boar - and more unusual bear, walrus, cougar, and fox). Prevalence depends on what meat is eaten and especially whether raw meat is eaten. The infection is rare in the US, but antibodies can, in some rare places, reach the double digits outside of the US.

There are other worms as well, such as schistosomiasis which spreads to bladders and around the liver and colon.

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