What are Obligate Parasites?

Michael Anissimov
Michael Anissimov

An obligate parasite is a parasite that must be with its host, or it dies. Obligate parasites depend on the presence of a host to complete their life cycle. Obligate parasites are common. There are parasitic plants, fungi, bacteria, and animals. The inverse of an obligate parasite is a facultative parasite, a parasite that can complete its life cycle independent of a host.

The Chlamydia bacterium is responsible for the #1 sexually transmitted disease in the world.
The Chlamydia bacterium is responsible for the #1 sexually transmitted disease in the world.

One of the most standard obligate parasites are viruses. Viruses are bits of genetic material covered in a protein sheath, capable of hijacking the protein synthesis machinery of cells and using them to pump out copies of the virus. Because of their inability to reproduce independently, viruses have sometimes been excluded from the kingdom of life, although this definition of "life" may be inappropriate because there are a number of more complex obligate parasites. Defending themselves from viruses may have been one of the first evolutionary imperatives of bacteria and eukaryotes, and both have evolved a range of error-checking genetic machinery and response mechanisms to slow down viral invaders.

There are other obligate intracellular parasites aside from viruses. These include bacteria like Chlamydias and Rickettsia, among the smallest viruses with the least complex genomes. The Chlamydia bacterium is responsible for the #1 sexually transmitted disease in the world, chlamydia, which is also the foremost cause of infectious blindness. Because obligate intracellular parasites have no tractable genetic system, and cannot be grown in a conventional artificial nutrient environments, and require a tissue culture, they can be difficult to study. Historically, these bacteria were considered to be organisms somewhere between viruses and bacteria.

Even some protozoa (eukaryotes, cells much more complex than bacteria) are obligate intracellular parasites, notably Plasmodium, at least ten species of which infect humans. These are thought to descend from dinoflagellates, photosynthetic protozoa, which eventually lost their photosynthetic ability as their parasitic lifestyle increased in emphasis. Interestingly, it is thought that mitochondria, the power stations present in every human cell, may have begun their evolutionary path as intracellular parasites, but subsequently became so integrated into the host that they actually became a part of it.

Michael Anissimov
Michael Anissimov

Michael is a longtime wiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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Discussion Comments


@PinkLady4 – A virus (and other obligate intracellular parasites) outside a host can survive, as we are obviously able to study them in laboratories without hosts, but it cannot reproduce without a host.

In certain environments, viruses can die if not within a host (such as in the freezing cold or at high temperatures – which is why refrigeration and baking are able to give you food free of viruses such as e. coli). However, there are other environments that are conducive to keeping virus cells alive until a new host arrives, at which point the virus can infect the host and then be able to reproduce again.

As for how many times they can reproduce, that just depends on the type of obligate intracellular parasite that has infected you, whether it has found a host cell compatible to reproduce its genetic material (as some parasites are very picky while others are not), and how long the virus is able to stay within the host.


I'm trying to figure out how viruses are obligate intracellular parasites, and also looking for more details about their reproduction. I realize that the only way they can reproduce is to enter or infect a host cell. Once established inside a cell, they can reproduce "offspring" who can carry out the same procedure again.

When a virus is not in a cell, are they in the dying process or what? How long can they live outside the host cell? How many times can they reproduce while they are in the host cell?


great explanation. helped me a great deal for my essay!


I'm studying for a Bio test and I needed a simple definition of an obligate parasite. I googled it and this page helped me a ton. It was very simple but I now know what it is. It answered a couple of my other questions, too.

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