lpetrich
Contributor
Infectious organisms have great variety, and they are scattered across the family tree of life. "Virus" is sometimes used as a generic term for them, even though only some infectious organisms are viruses, but "bug" is often used informally, even by biologists and medical personnel.
Viruses
Viruses depend on their host cells to reproduce, and outside their host cells, they are genetic material surrounded by a protein coat. Something that is sometimes called a "virion". Some viruses can insert themselves into the genomes of their host cells, thus becoming "slow viruses".
Some biologists argue about whether they should be called "alive".
Viruses have a lot of variety in their genomes.
There are three main theories of virus origin:
Viruses are unaffected by medicines that attack cellular organisms, but some antiviral ones do exist. They work by attacking the virus when it is reproducing in its host cell.
Bacteria
These are cellular organisms, and nearly all of them are one-celled or undifferentiated multicelled ones. They are prokaryotes, organisms without a well-defined cell nucleus.
Their phylogeny was mysterious for a long time, and was only cracked with the development of gene sequencing. Their phylogeny has the complication that many of them have exchanged many genes, and some biologists have proposed that a family-tree model does not work very well with them. However, one can get a well-defined phylogeny out of information-system genes, even if not for metabolic-system genes.
The oldest known divergence in our planet's biota is between the Bacteria (ordinary bacteria) and Archaea. The Bacteria are in turn subdivided into several phyla, and disease organisms are scattered over them, though notably in Proteobacteria (Salmonella, Vibrio cholerae), Firmicutes (Clostridium spp., Bacillus anthracis, Streptococcus pyogenes), and Actinobacteria (Mycobacterium tuberculosis).
One-Celled Eukaryotes
These include various one-celled fungi, like yeasts, and also a variety of protists. "Protist" is a garbage-can taxon for one-celled and primitive multicelled eukaryotes. In any case, disease organisms are scattered among them, notably coming from Fungi (the yeast Candida spp.), Alveolata (malaria bug Plasmodium falciparum), Excavata (Giardia lamblia, Trichomonas vaginalis), etc.
Multicelled Eukaryotes
These include various flatworms (platyhelminthes: flukes, tapeworms) and roundworms (nematodes), all in the animal kingdom.
Plants have a variety of diseases caused by multicelled fungi. These include the "fungus" Phytophthora infestans. This is not a strict-sense fungus, but more closely related to kelp and diatom algae (Stramenopiles). It grows like a fungus, however, a case of convergent evolution. Plants also suffer from parasitic plants. These also grow like fungi. Other organisms with a fungus habit include rhizocephalans like Sacculina, a parasitic barnacle that lives on crabs, and some actinobacteria, formerly called actinomycetes.
Viruses
Viruses depend on their host cells to reproduce, and outside their host cells, they are genetic material surrounded by a protein coat. Something that is sometimes called a "virion". Some viruses can insert themselves into the genomes of their host cells, thus becoming "slow viruses".
Some biologists argue about whether they should be called "alive".
Viruses have a lot of variety in their genomes.
- DNA, RNA
- Single-strand, double-strand
- For single strand: positive sense, negative sense, mixed sense
There are three main theories of virus origin:
- Primordial organisms, emerging when the first cellular organisms emerged
- Transposable elements (transposons, "jumping genes") and plasmids (loose bits of genetic material) that became able to depart from their host cells
- Heavily reduced cellular organisms
Viruses are unaffected by medicines that attack cellular organisms, but some antiviral ones do exist. They work by attacking the virus when it is reproducing in its host cell.
Bacteria
These are cellular organisms, and nearly all of them are one-celled or undifferentiated multicelled ones. They are prokaryotes, organisms without a well-defined cell nucleus.
Their phylogeny was mysterious for a long time, and was only cracked with the development of gene sequencing. Their phylogeny has the complication that many of them have exchanged many genes, and some biologists have proposed that a family-tree model does not work very well with them. However, one can get a well-defined phylogeny out of information-system genes, even if not for metabolic-system genes.
The oldest known divergence in our planet's biota is between the Bacteria (ordinary bacteria) and Archaea. The Bacteria are in turn subdivided into several phyla, and disease organisms are scattered over them, though notably in Proteobacteria (Salmonella, Vibrio cholerae), Firmicutes (Clostridium spp., Bacillus anthracis, Streptococcus pyogenes), and Actinobacteria (Mycobacterium tuberculosis).
One-Celled Eukaryotes
These include various one-celled fungi, like yeasts, and also a variety of protists. "Protist" is a garbage-can taxon for one-celled and primitive multicelled eukaryotes. In any case, disease organisms are scattered among them, notably coming from Fungi (the yeast Candida spp.), Alveolata (malaria bug Plasmodium falciparum), Excavata (Giardia lamblia, Trichomonas vaginalis), etc.
Multicelled Eukaryotes
These include various flatworms (platyhelminthes: flukes, tapeworms) and roundworms (nematodes), all in the animal kingdom.
Plants have a variety of diseases caused by multicelled fungi. These include the "fungus" Phytophthora infestans. This is not a strict-sense fungus, but more closely related to kelp and diatom algae (Stramenopiles). It grows like a fungus, however, a case of convergent evolution. Plants also suffer from parasitic plants. These also grow like fungi. Other organisms with a fungus habit include rhizocephalans like Sacculina, a parasitic barnacle that lives on crabs, and some actinobacteria, formerly called actinomycetes.