Single Celled Organisms | A List with Examples including Few Parasites

An organism is one which lives an independent life.

Single-cell organisms are quite interesting in that with a  single cell, they perform the entire activities.

There are some billions of cells in the human body that make up tissues and further organs to function.

In a single-cell organism, the cell is so designed to carry out all the life activities with it.

However, there are few important modifications that make it sustain so well.

Single-Celled Organisms


Vorticella a protozoan

These are the eukaryotic cells that live mostly in water. It can be freshwater or marine water but very rarely in soil or land. They are many types of protozoa and some are free-living while others act as parasites. Some of them even cause severe diseases in humans.

They generally consist of a plasma membrane, nucleus, mitochondria, vesicles, flagella, cilia and other cell organelles. Though most of them free living, they get their food from external sources. They eat up bacteria and other organic matter.

Examples include Amoeba, euglena, paramecium.

Parasites include: Entamoeba, plasmodium


These are prokaryotic single-celled organisms. They are said to be omnipresent. They are found in water, soil, air and even in extreme climates like heat and cold. There are many disease-causing bacteria and have been affecting humans for centuries. These bacteria have the ability to derive energy from multiple sources. Some of them are parasites, some saprophytes, autotrophs and even chemotrophs.

Rod shaped bacteria

Examples include E-coli, staphylococcus bacteria, salmonella typhi, pseudomonas aerogenosa etc.


These are another set of prokaryotes that significantly differ from bacteria. There are anaerobic organisms which means they live in areas with no oxygen. They are found in harsh climates like hot springs, saltwater. Besides they are also found in soils and oceans.

Single-celled parasite

A parasite is an organism that survives on another organism and derives nutrition and other requirements for living at the expense of the host.

Hence, the host organism suffers due to the parasite attack on it.

There are many types of parasites based on their preferences, modes of transmission, etc.

But those with shape and structure are quite interesting.

Few of the Microorganisms are the parasites which have small structure. They can be either single-celled or multi-celled parasites. But in this article, we will see the list of single-celled parasites and their behaviors.

These single-celled parasites are mostly either protozoa or bacteria. They cause diseases to humans, animals, and plants. They are widely present across all parts of the world and can also become epidemic at a time leading to mass infections and deaths.

They can gain entry into humans through food, water, air and by contact. Inside the humans and other organisms, they live by feeding the tissues, cells and other essentials and multiply rapidly leading to sickness, diseases in the host. If not controlled, they can sometimes lead to the death of the host.

Examples of the single-celled parasite in humans and their features

1. Protozoan single-celled parasite

Protozoans exist as parasites in the human body. Some of them stay for a very long time in the body without being noticed except for minute health issues. Example includes

Entameba histolytica

This a common pathogenic protozoan that relies on the human intestine for a long time. It can also pass on to other body organs like the liver, heart and even brain. In the intestine, it causes symptoms like pain, mucus stools, etc.

Entamoeba in the intestine by CDC

But if the population is more, the pain can be severe and need medical attention. Since it is an amoeba, it is called a shape altering single-celled parasite.

Toxoplasma gonodi

This is an interesting protozoan that exists in the blood of man transmitted from cat through food and water contamination. It almost shows negligible symptoms in man for a very long time. And it is said to infect half of the human population in the world.Single Celled Parasite-taxoplasma parasite

It reproduces in cat and spread mostly by improperly cooked meat, dairy products, etc.

Single Celled Parasite-malaria
Malarial parasite Plasmodium inside the red blood cells

Malaria is a highly contagious protozoan parasite transmitted to humans by the female Anopheles mosquito (insect). They are of 4 types of parasites as plasmodium vivox, falciparum, malaria and ovale. The symptoms of infection include repeated short duration fevers, chills, headaches, etc.  If the parasite life cycle is not controlled by drug treatment, it leads to death.

In the year 2016, there were 216 million cases of malaria infection worldwide of which 445,000 people died. In the United States each year there are approximately 1,700 cases of diagnosed with malaria especially among tourists.

Cryptosporidium is another protozoan that infects animals and humans. It is mostly transmitted by water. It causes diarrhea as it lives in the intestine.

Giardia is a ciliated protozoan parasite that causes giardiasis. It is spread by contaminated drinking water.

Leishmaniasis is a flagellate protozoan parasitic spread by the bite of a sandfly. The common symptoms are which causes skin sores, mucous ulcers in the nose, mouth, fever, anemia, liver, spleen and bone marrow damage.

Balantidium coli is a ciliated protozoan parasite. It infects the intestines of humans. It is transmitted through the contaminated food and water. Symptoms include abdominal pain and diarrhea

2. Bacteria parasites

A parasite is one that stays in the host body and cannot live on its own without a host. But, most bacteria can be grown in lab cultures out of the host. So they are not classified as parasites but as pathogens. However, few bacteria are not grown in lab cultures making them classified as a parasite.

Mycobacterium leprae: This is a dreaded bacteria as per the world health organization and it has threatened the future existence of mankind in the past. This is a bacteria that can easily spread from an infected person to another just by air through salivary droplets.

These bacteria once entered into the human will stay in the body for a lifetime. However, it is discovered that these bacteria have lost the capacity to be grown in lab cultures. This is because their metabolic system is completely changed to just survive in the host.

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  1. It seems the earth is full of living organisms so there is no empty space.
    Ie there is no void, vacuous spaces
    or total emptiness on earth.


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