As if bacteria and viruses weren't enough, there are other creepy crawlies out there that are pathogens and cause disease. They're less common than bacteria and viruses, but they can be just as deadly. Yes, fungi and parasites are also potential disease-causing agents.

Fungi

Fungi are multicellular organisms in the Eukarya domain. They're found all over, in soil, on vegetation, and even on parts of our bodies. Mushrooms are probably the most recognizable type of fungi, made popular by the Super Mario Brothers. Fungi reproduce by sporulation, which is the process of releasing spores out into the world and then hoping they go on to live productive lives.

Many fungi are harmless. For example, penicillin is a fungus that produces a powerful antibiotic. Yeast is a type of fungi used to make beer, bread, and wine. And mushrooms are just plain delicious. There are a handful of fungi that like to cause problems, but we try not to hang out with them.


Bread: a better gift for that fungi lover than athlete's foot.

Yet, while some fungi can make some pretty scrumptious grub, other types of fungi can be just as harmful as vicious types of bacteria and viruses. You can also call fungal infections mycotic infections, if you feel like working out your pretentious muscle.

Just like other infections, the symptoms of a fungal infection differ depending on which fungus is doing the infecting and which host it's crashing. If a drug treats a fungal infection, it's called an antifungal. Below are some descriptions of infectious diseases caused by fungi.

Aspergillosis

Aspergillosis is a disease caused by an infection with the fungus Aspergillus. It's that thing that looks like an underwater flower in the picture below. It affects the respiratory system of the body, and in severe cases it can destroy body tissues. Aspergillus is typically encountered in the environment in soil, plants, or dust.


Aspergillus. Image from here.

Yeast Infection and Thrush

Both yeast infections and thrush are caused by infection with one of 20 yeasts of the family Candida. For this reason, these diseases are also known as candidiasis. Trust us, it is nothing like candy.

This type of yeast is commonly present on the skin and in the mucosa, but when they overgrow they cause infectious disease. Overgrowth in the vagina causes itching, burning, and irregular discharge. Approximately 75% of women will have a vaginal yeast infection at some point in their lives. Overgrowth in the mouth causes white patches or plaques to form in the mouth called thrush.

Both can be treated with anti-fungal drugs. If the yeast gets into the bloodstream, a more invasive form of candidiasis can occur, which causes fever, chills, and possible organ failure depending on where the infection spreads.

Ringworm, Jock Itch, Athlete's Foot

Ringworm, jock itch, and athlete's foot are all caused by infection by a group of fungi called dermatophytes or tinea fungus (the most common of which is a fungus called Trichophyton, which also sounds like the name of a Decepticon). These fungi live on moist skin (like in your armpits and groin and between your toes) and can live on clothing and bedding as well. Blech. Kinda makes you wanna wash your bedsheets right now, huh?

Common symptoms are itchiness, redness, scaling, cracking of the skin, and loss of hair. A ring-shaped rash may occur, and affected nails can become thick and discolored.

Ringworm can occur in many places and is often caught from infected pets. Jock itch refers to infection of the skin in the genital area. Athlete's foot is infection on the feet, generally between the toes. This is exactly why nixing the flip-flops in a public shower or locker room is never a good idea.


Athlete's foot between the toes. Image from here.

Parasites

A parasite is an organism that is mooching a free ride from another organism. They get their food and energy from the host and often cause disease.

Parasites are totally creepy to think about. They're basically a tiny creature living inside you like an unwanted houseguest. But hey, you already have trillions of bacteria living inside of you—what's all the fuss about?

Parasites are eukaryotes and are larger than a unicellular bacteria, so this probably adds to the creep-o-meter. They can be unicellular, like amoebas or cryptosporidium, or multicellular. There are rumors about tapeworms more than 20 feet long. Seriously. And believe it or not, some people infect themselves with tapeworm because it might help with a shrinking waistline. We'll just take a salad, minus the tapeworm larvae. Below is more information about a few parasites, so you can fully understand what's in your nightmares tonight.

Hookworm, Roundworm, Whipworm

Unlike ringworm (which is not caused by a worm), these are all infections of soil-transmitted helminths…y'know, actual worms. A different species of helminth causes each infection.

Hookworms can be contracted in soil that is contaminated with human feces. The tiny worms then penetrate the skin and migrate to the intestine. Do you have the heebie jeebies yet? Hookworms can cause anemia and malnutrition. 567 million people are infected with hookworm. That is 1 out of every 12 people on the planet.


Life cycle of hookworm. Image from here.

Roundworms can be contracted by eating something contaminated with human feces (yum). They can grow up to 12 inches long and can live for 1–2 years in the intestine. Roundworm causes the most human infections of all the helminths: 1 out of 9 people are infected with roundworms and 60,000 people die each year.

Whipworm affects 1 out of 12 people and is contracted by eating something contaminated with human feces. (Notice a trend here? Don't eat poop.) Whipworm females can lay up to 20,000 eggs per day. If given the choice between whipped cream or whipworm, we at Shmoop choose whipped cream.

African Eyeworm

African eyeworm is one of the creepier parasites out there, but it is not very deadly. It is caused by infection with the Loa loa worm. Whatta whatta? Loa loa.

The worm is carried in the African deerfly, which transmits the parasite to humans through its bite. Although the worm travels to other places in the body, passages across the eye are quite noticeable. Apart from visualization of the worms, other symptoms are irritation or itchy swellings of the skin.

The worms can be surgically removed, but drug treatment is also required to kill the larvae, or baby worms. This disease is most prevalent in Central Western Africa, where it's estimated that 13 million people are infected.


A 5cm Loa loa worm extracted from a patient's eyeball. ~shudder~ Image from here.

Malaria

The deadliest parasite in the world is Plasmodium, which causes malaria. Plasmodium is carried by mosquitos and then transferred to humans through their bite. Plasmodium replicates in the liver and can later infect red blood cells.

Malaria causes fever, chills, and fatigue, and can cause serious complications leading to death. Approximately 216 million cases and 655,000 deaths were estimated from malaria in 2011, mostly in children in Africa where the disease is prevalent.

Malaria can be treated with drugs, although drug resistance is on the rise. An international campaign to provide mosquito netting and insecticide is currently underway to reduce malaria exposure.


Children in Senegal demonstrating how to properly use a mosquito net to prevent malaria. Image from here.

Toxoplasmosis

Approximately 1 out of 5 people in the United States is infected with the parasite Toxoplasma gondii (T. gondii), which causes toxoplasmosis. Many people don't know they're infected; they either show no symptoms or they recover quickly. It can, however, be a problem for pregnant women or immunocompromised people.

T. gondii can be found in undercooked meat, water, and cat feces. Meow. Symptoms can affect the brain, eyes, and other organs. Pregnant women are advised to steer clear of Fluffy's litter box, especially early in pregnancy. Any contact with cat feces poses a threat to the fetus. Babies born to women who became infected early in pregnancy are at risk for blindness later in life and mental disabilities.

"Brain Eating Amoeba"

This disease is just as scary as it sounds, and it's caused by the parasite Naegleria fowleri. This parasite is commonly found in warm bodies of freshwater. Drinking the water is safe, but if the amoeba gets into your nose, it is bad news. From there, the parasite travels to the brain, where it takes up residence, destroys the brain tissue, and causes brain swelling that leads to death.

It's a very rare infection, with less than five cases per year in the United States. Recently, several people became infected when using unclean water in a neti pot to irrigate their noses. Other than neti pots, people primarily become infected after swimming or submerging their nose in a contaminated body of water. Since it can only infect you through your nose, nose plugs are probably a safe bet.


Naegleria fowleri (stained green) inside brain tissue. Think it's pretty? You're right…pretty dangerous. Image from here.

Brain Snack

We've discussed helminths as a deadly parasite, but they can also be a therapeutic strategy. What? That's right; some people choose to infect themselves with worms because they believe it's beneficial to help with bowel disorders, allergies, and asthma.

You're probably wondering how a tiny parasitic worm could help cure so many ailments. Most of the worms we're familiar with don't know their head from their tail. Well, it's not exactly the worm that's got the magic healing powers, it's your immune system.

Scientists and doctors believe that in today's hyper-clean society, where everything is doused with bleach and antibiotics are prescribed to people who don't need them, our bodes don't get exposed to pathogens as much anymore. You might think this is a good thing, but your immune system is screaming, "We're booooored!" The immune system likes to stay active, and when we take away all of its pathogen playmates, it goes looking to pick a fight, sometimes with our own body's cells. This results in things like allergies, asthma, and autoimmune disorders.

It's thought that throwing a parasitic worm in the mix causes changes in the immune system that lessen the symptoms of problems like asthma and allergies. Sounds like a pretty good deal, right? There are a few drawbacks. Remember, parasites aren't in it for your health; they're in it for their own, which means they're using your flesh as a buffet. Individuals housing these worms can develop anemia and can have reactions to the worm that include itching, swelling, rashes, and digestive problems.

Of course more research needs to be conducted to find out how exactly helminthic therapy works and any long-term side effects. Scientists and doctors are also looking at ways to replicate the benefits of these parasitic worms without having to, you know, put a parasitic worm in someone's body.


Hookworms attached to the intestine. Image from here.