When you think of a parasite, do you think of a tapeworm living in the small intestine? A guinea worm erupting from a hand or foot? Or perhaps fleas or head lice jumping from dog to dog, kid to kid? While those are all terrifyingly fantastic, there is another type of parasite that exists, unseen, all around us. I’m talking about an obligate intracellular parasite, also known as a virus.

Catching these all at once is not advised (clockwise from top left: Rhinovirus, West Nile virus, rabies, rubella)

Not quite alive, but also not completely inanimate, viruses need a host to grow and reproduce. Their structures are simple: some DNA or RNA encased in a protein shell, covered in a layer of lipids with glycoprotein knobs sticking out. In the environment, they float or drift or roll around like empty cardboard boxes on a windy day. When they encounter a cell with the exact surface proteins for them to attach, they lock into place and inject their genetic material into the unsuspecting cell.

From here, the viral DNA is incorporated into the host cell’s genome. If the virus contains RNA, the RNA is first turned into DNA and then inserted. Either way, the host cell has been taken over. It is “forced*” to make thousands of viruses and eventually, the cell explodes. Boom! Now these new viruses are ready to go out into the world and infect other healthy cells.

Did I scare you? Don’t be scared. Most viruses don’t infect people. Because their attachment pieces are glycoproteins, viruses are extremely picky as to the type of cell they stick to. You can’t give your dog a cold and your dog can’t give you parvovirus. So cuddle away with Fido next time you catch a cold.
Of course, there are always exceptions. Some viruses, such as rabies, can infect groups of animals. Ever read the book Old Yeller? The reason for its tragic ending was due to the fact that rabies can be transmitted from animals to humans (and vice versa); in fact, it is a virus that can be transmitted between all mammals, but if you were a turtle, you’d be okay.

Finally, there is a special group of viruses that exclusively targets prokaryotes (bacteria) called bacteriophages or phages. Yes, scientists gave them a nickname. While this may seem trivial and unimportant, remember that you are made up of more bacterial cells than your own cells. In fact, the story of phages is so interesting and affects our lives so much that it deserves its own post (here).

Define Parasite
Parasites come in all shapes and sizes. Just like with groups of people, it’s best not to stereotype them. The best definition of a parasite that I heard was from my college parasitology professor. He told us that a parasite is an organism that needs the nutrients and resources of another organism (of a different species) to survive. He also said that the clause in parenthesis about the parasite needing to be a different species from its host is needed so that fetuses are not categorized as parasites. I’ll now end this post and let you reflect on that.

*I used quotes around the word forced because the cell isn’t actually being held at gun point by the virus, it’s just not making its own proteins anymore.

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