Difference Between Pathogen and Parasite in Biology

Pathogens and parasites are two terms that describe microorganisms that can be harmful to the health of any living being. But the major difference between pathogen and parasite is that the term pathogen describes the effect of a microorganism on another living being which is often harmful, whereas the term parasite describes the mode of living of a microorganism which may or may not be harmful. This becomes more clear as we look at the definitions: 

• Pathogen: 

A microorganism that causes disease after infection can be called a pathogen. It is important to note that any microorganism that causes disease after infecting another organism, which is usually detrimental in nature, then and only then, the said microorganism can be called a pathogen. 

• Parasite:

A microorganism that infects other living organisms and derives benefits such as nutrients from the host which may or may not cause a decrease in the fitness of the host can be called a parasite. Unlike pathogens, it is a necessity for a parasite to be attached to living inside or alongside another organism i.e. for a parasite to survive it is essential that it infect another organism, in general terms. But this infection may or may not lead to the deterioration of the health and fitness of the host organism. This becomes as mentioned earlier, quite a prima facie difference between pathogens and parasites.

What is the Difference Between Pathogen and Parasite?

Pathogens and Parasites: A more detailed view 

It’s quite easy for anyone to have a mix between the concept of pathogens and parasites. But as stated above, a clear difference between pathogen and parasite exists as a pathogen is the cause of a disease, whereas the same may not necessarily be the case for a parasite. For example, Salmonella typhi is a pathogenic bacteria that cause typhoid fever, whereas, on the other hand, some bacteria can prevent colonisation by microorganisms such as Salmonella enterica - a bacteria that causes food poisoning, thus protecting the human body.  

In another view, it can be easily considered that pathogens are the causative agents of any diseases, whereas parasites may or may not provide these causative agents after infection to the host. As a parasite is dependent on its host to survive its life cycle, it may help the host to survive as well. But this is not necessarily true for every parasite. Some may exchange such benefits whilst others do not. Also, parasites can be pathogenic in nature when they release the causative agents of diseases into the host cell, but pathogens are never necessarily parasites since pathogens can have their own independent life cycle. As mentioned above, bacteria, fungi, protozoa and viruses can be pathogenic and parasitic, both except for the viruses. Viruses are usually pathogenic and not parasitic. 

To conclude, it can be simply stated that any microorganism that can cause the degradation of the health and fitness of another living organism owing to the processes of its own life cycle, can be called a pathogen; whereas, any microorganism that essentially requires another organism to carry out its essential functions during its life cycle can be called a parasite. Hence, one is primarily an effect of life stages and the other is primarily a requirement of the benefits for the life stages.

Parasites vs Pathogens

Parasites

Parasites are living organisms which live inside or attached to other organisms to draw some benefit from it. The organism on which a parasite lives is called a host. The parasite's life depends on attachment with the host and without the host the parasite will lose its life. Some common examples of parasites are Fungi, lice, ticks, tapeworms and pinworms. Sometimes the parasite causes infection in the host which could also be severe and may cause death of the parasite. 

In most cases parasites depend on the host for nutrition and shelter. Without the host the parasite will die. Some of the parasites manipulate/infect the host in a way that the host will provide the nutrients and shelter to it which is out of the host's natural behavior. This causes the health of the host to deteriorate and may be fatal in some cases.

Most of the parasites are not harmful in nature. In fact some of the parasites cause benefits to the host. In the human body there are parasitic bacteria in our intestine which helps us in digestion of our food and that results in better absorption of nutrients from our food. Few parasitic bacteria also help us in preventing infection from disease causing parasites. The parasite depends on the host to live the whole life so it is always in the best interest of the parasite that the host lives a healthy life. The healthy host will always provide better food and nutrients to the parasite. But remember it is not always the case and parasites can cause disease directly or indirectly to the host organism.

Pathogens

Pathogens are organisms which cause disease in other organisms after infecting them. The pathogens depend on the host organism to achieve some of its life functions and while achieving that it causes the degradation of health for the host. Some common examples of pathogens are Bacteria, Protozoa, Virus and Fungi.

Some pathogens are parasitic in nature and may depend on the host to achieve its nutrients and shelter but they will be called pathogens only if they cause disease in the host. 

All the transmissible diseases in animals are caused by pathogens. Pathogens are by nature harmful to the host and they normally depend on the host to complete one phase of their life cycle.

For example the virus enters the host organism's cell and reproduces its progeny inside it causing the cell to rupture. After that the virus moves out to infect new cells.