- Explain how protists cause human disease.
- Identify three ways fungi can make humans sick.
infection of the skin between the toes by the fungus Trichophyton
infection of the mouth or of the vagina in females that is caused by the yeast Candida
disease caused by Giardia protozoa that spreads through contaminated food or water
disease caused by Plasmodium protozoa and transmitted by mosquitoes in tropical and subtropical regions of the world
skin infection caused by the fungus Trichophyton that causes a characteristic ring-shaped rash
Protists and fungi may seem defenseless, but they can be deadly. Both are important causes of disease and death in other living things—including humans.
Protists and Human Disease
Most protist diseases in humans are caused by animal-like protists, or protozoa. Protozoa make us sick when they become human parasites. Three examples of parasitic protozoa are described below.
Members of the genus Trypanosoma are flagellate protozoa that cause sleeping sickness, which is common in Africa. They also cause Chagas disease, which is common in South America. The parasites are spread by insect vectors. The vector for Chagas disease is shown in Figure below. Trypanosoma parasites enter a person’s blood when the vector bites. Then they spread to other tissues and organs. The diseases may be fatal without medical treatment.
Vector for Chagas Disease. In Chagas disease, the Trypanosoma parasite is spread by an insect commonly called the “kissing bug.” A bite from this bug could be the kiss of death.
The discovery of Chagas disease is unique in the history of medicine. That’s because a single researcher—a Brazilian physician named Carlos Chagas—single-handedly identified and explained the new infectious disease. In the early 1900s, Chagas did careful lab and field studies. He determined the pathogen, vector, host, symptoms, and mode of transmission of the disease that is now named for him.
Giardia are flagellate protozoa that cause giardiasis. The parasites enter the body through food or water that has been contaminated by feces of infected people or animals. The protozoa attach to the lining of the host’s small intestine, where they prevent the host from fully absorbing nutrients. They may also cause diarrhea, abdominal pain, and fever. A picture of a Giardia protozoan opens this chapter.
Plasmodium protozoa cause malaria. The parasites are spread by a mosquito vector. Parasites enter a host’s blood through the bite of an infected mosquito. The parasites infect the host’s red blood cells, causing symptoms such as fever, joint pain, anemia, and fatigue.
Malaria is common in tropical and subtropical climates throughout the world (see Figure below). In fact, malaria is one of the most common infectious diseases on the planet. Malaria is also a very serious disease. It kills several million people each year, most of them children.
Worldwide Distribution of Malaria. This map shows where malaria is found. The area is determined by the mosquito vector. The mosquito can live year-round only in the red-shaded areas.
Fungi and Human Disease
Fungi cause human illness in three different ways: poisonings, parasitic infections, and allergic reactions.
Many fungi protect themselves from parasites and predators by producing toxic chemicals. If people eat toxic fungi, they may experience digestive problems, hallucinations, organ failure, and even death. Most cases of mushroom poisoning are due to mistaken identity. That’s because many toxic mushrooms look very similar to safe, edible mushrooms. An example is shown in Figure below.
Poisonous or Edible? The destroying angel mushroom on the left causes liver and kidney failure. The puffball mushroom on the right is tasty and harmless. Do you think you could tell these two species of mushrooms apart?
Some fungi cause disease when they become human parasites. Two examples are fungi in the genera Candida and Trichophyton.
Candida are yeast that cause candidiasis, commonly called a “yeast infection.” The yeast can infect the mouth or the vagina (in females). If yeast enter the blood, they cause a potentially life threatening illness. However, this is rare, except in people with a depressed immune system.
Trichophyton are fungi that cause ringworm. This is a skin infection characterized by a ring-shaped rash. The rash may occur on the arms, legs, head, neck, or trunk. The same fungi cause athlete’s foot when they infect the skin between the toes. Athlete’s foot is the second most common skin disease in the U.S.
Figure below shows signs of these two infections.
Ringworm produces a ring-shaped rash, but it isn’t caused by a worm. It’s caused by the same fungus that causes athlete’s foot.
Mold allergies are very common. They are caused by airborne mold spores. When the spores enter the respiratory tract, the immune system responds to them as though they were harmful microbes. Symptoms may include sneezing, coughing, and difficulty breathing. The symptoms are likely to be more severe in people with asthma or other respiratory diseases. Long-term exposure to mold spores may also weaken the immune system.
Molds grow indoors as well as out. Indoors, they grow in showers, basements, and other damp places. Homes damaged in floods and hurricanes may have mold growing just about everywhere (see Figure below). Indoor mold may cause more health problems than outdoor mold because of the closed, confined space. Most people also spend more time indoors than out.
The mold growing on the walls and ceiling of this storm-damaged home may be harmful to human health.
- Most protist diseases in humans are caused by protozoa. Protozoa make humans sick when they become human parasites. Trypanosoma protozoa cause Chagas disease and sleeping sickness. Giardia protozoa cause giardiasis, and Plasmodium protozoa cause malaria.
- Fungi cause three different types of human illness: poisonings, parasitic infections, and allergies. Many poisonous mushrooms are eaten by mistake because they look like edible mushrooms. Parasitic yeasts cause candidiasis, ringworm, and athlete’s foot. Mold allergies are very common.
Lesson Review Questions
1. Describe how the protozoa that cause Chagas disease are spread to human hosts.
2. State why malaria is commonly found only in tropical and subtropical regions of the world.
3. How does mold cause allergies?
4. State why indoor mold may cause more health problems than outdoor mold.
5. Terri lost her water bottle while hiking in Canada. It was a hot day, so she drank water from a stream to stay hydrated. A few days later, Terri became ill with abdominal pain, fever, and diarrhea. Her doctor thinks she has a protozoan infection. Which type of protozoa do you think is most likely responsible for Terri’s illness? How do you think Terri became infected?
6. Explain why you should never eat mushrooms you find in the woods unless you know for certain which type of mushrooms they are.
7. Compare and contrast ringworm and athlete’s foot.
Points to Consider
In this chapter you learned about two kingdoms of Eukarya, the protists and fungi. In the next chapter, you’ll learn about another kingdom of Eukarya, the plants.
- Plants are a very diverse kingdom. How many different kinds of plants can you think of?
- What traits do you think might distinguish plants from other eukaryotes? What do you already know about plants that might help you answer this question?
Opening image copyright Sebastian Kaulitzki, 2010. Used under license from Shutterstock.com.
For Table above, from top to bottom,
For Table above, from top to bottom,
For Table above, from top to bottom,