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I found the offensive word, thanks to MCXCI. (YOU'RE A HERO, MAN).
We'll mainly be classifying and distinguishing different disease types, because that still seems to be something people are having trouble doing. This isn't including genetic diseases; only bacteria, parasites, viruses, and a bonus type. It requires only a basic understanding, so I won't go on for too long about it. We'll just start with the various traits of each. Before we start, it's important to make this very clear; bacteria are a type of parasite. I'll cover that more later.
I haven't really made many epidemiology threads since the EVD scare of 2014, so it's good to get back to my roots. Being my first and favorite science field, this is one topic I could go on forever about. I hope to make this into a career, so I should get used to talking about it.
Prepare for a huge wall of text.
1. Bacteria:
Bacteria are prokaryotic parasites in most cases; at least when referring to infectious diseases. They live symbiotically, but some can also live on their own. Parasitic bacteria are symbiotic.
There are five different types of bacteria characterize by their shape. Bacilli, cocci, spirilla, rickettsia, and mycoplasma. Bacilli are almost cylindrical, cocci are spherical, and spirilla are curved. Rickettsia and mycoplasma aren't characterized by shape like the other three. Rickettsia are rod-shaped, but can also be spherical; they are a variation of bacilli or cocci. Mycoplasma are typically cocci, but they lack a cell wall.
The organelles in most bacteria are as follows: cell wall, plasma membrane, cytoplasm, ribosome, plasmid, flagella, pilli, and a capsule. Some bacteria (such as mycoplasma) don't actually have a cell wall, so these organelles aren't universally present in bacteria. Most bacteria have them though.
2. Parasites:
I won't spend too much time on parasites, as I covered most of their properties in bacteria section. Parasite is just a colloquial term for a unicellular or multicellular organism that uses other animals for nutrients. Some protozoa and helminths can be free-living.
Protozoa are single-celled, but they cluster together in a way that would make it appear as though they are multicellular. They can be free-living or a symbiotic parasite, but their ability to reproduce within humans is astounding. They are typically transferred via arthropod vector. For example, the type of malaria that infects humans is caused by four different types of the protozoan parasite plasmodium; falciparum, ovale, malariae, and vivax. Sometimes species like P. knowlesi will also infect humans.
Helminths are multicellular, snake-like parasites. They are so large that they are actually visible to the naked eye in their adult stages. They can be symbiotic or free-living. Unlike protozoa, once they are in their adult form they cannot multiply inside of a host. The hosts are simply there for nutrients. There are three different types of helminths. Platyhelminths (flatworms), acanthocephalins (horn-headed worms), and nematodes (roundworms). I won't go too much into the characteristics of each, it would take way too long.
Ectoparasites are, as the name implies, external parasites. They include things like hematophagic arthropods, such as mosquitoes. Ectoparasitism is usually used to classify things like lice, fleas, mites, and ticks, however. Hematophagic arthropods only stay for a matter of seconds, whereas the latter stay for days, weeks, or even months. Ectoparasites aren't typically dangerous on their own, but they are very commonly used as vectors for more deadly diseases.
I spent way more time on that one than I intended.
3. Viruses:
Viruses are, by far, my favorite on this list. They're the most unpredictable, and they include some of the deadliest diseases we've ever seen.
There are way too many virus types to list here; it's not as simple as narrowing it down to protozoa and helminths or bacilli, cocci, spirilla. Most viruses are either helical or polyhedral, but the shapes vary greatly. Instead of focusing on that, I'll focus on the overall characteristics of viruses. More specifically, I will address why they're classified as nonliving, and what makes them different from parasites and bacteria.
Below, I will list a basic comprehensive list of the characteristics of viruses.
-Viruses are non-cellular.
-Viruses do not have a metabolic system.
-Viruses do not grow in size.
-Viruses reproduce, but not without the aid of a host.
-Viruses do not have ribosomes or enzymes required for metabolism.
-Viruses are contained inside of a capsid.
-In most cases, viruses have a protective envelope.
-Viruses have a core of nucleic acid, which can contain either DNA or RNA.
As you can see, viruses fall short of many of the characteristics of typical life. They are parasitic, but they are nonliving.
[BONUS] Prions:
Prions are not diseases in themselves. They are the result of a misfolded PrP, which causes other PrPs in your body to become misfolded. They are even less living than viruses, as they are simply just a rogue protein. They stimulate healthy PrPs, causing them to fold into PrP scrapie.
Misfolded prions are typically progressive neurodegenerative disorders, and they can affect both humans and non-human animals. Bovine spongiform encephalopathy (colloquially known as mad cow disease) is the most notable of this family.
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