Acellular Infectious Agents
Beyond cellular life, there exist even simpler entities that can cause disease: viruses and prions. These are not considered living organisms because they lack cellular structure and cannot reproduce on their own.
Viruses
A virus is an obligate intracellular parasite, essentially a package of genetic material in a protein shell.
Structure: All viruses have two basic components:
1.Genetic Material: Either DNA or RNA, which can be single-stranded or double-stranded.
2.Capsid: A protein coat that surrounds and protects the genetic material.
Some viruses also have an outer lipid envelope derived from the host cell's membrane.
Replication: Viruses cannot replicate independently. They must infect a living host cell and hijack its metabolic machinery to produce new virus particles (virions). There are two main replication cycles:
1.Lytic Cycle: The virus immediately takes over the host cell, forcing it to produce new viruses. The host cell then lyses (bursts), releasing the new virions to infect other cells. This results in the rapid death of the host cell and acute symptoms.
2.Lysogenic Cycle: The viral genetic material integrates itself into the host cell's chromosome, becoming a prophage (or provirus). The viral DNA is then replicated along with the host's DNA every time the cell divides, without harming the cell. An environmental trigger can cause the prophage to exit the host chromosome and enter the lytic cycle.
A retrovirus, such as HIV, is an RNA virus that replicates via a lysogenic cycle. It uses a special enzyme called reverse transcriptase to transcribe its RNA genome into DNA, which is then inserted into the host's chromosome.
Prions
Prions are an even simpler and more bizarre infectious agent. A prion is an infectious protein.
Mechanism: Prions are misfolded versions of a normal protein found in the brain. When a prion comes into contact with its normal, correctly folded counterpart, it induces the normal protein to misfold into the infectious prion form.
Effect: This sets off a chain reaction that leads to the accumulation of misfolded proteins, forming aggregates that cause brain tissue to become spongy and degenerate.
Diseases: Prion diseases, such as Creutzfeldt-Jakob disease in humans and "Mad Cow Disease" in cattle, are untreatable and universally fatal.