Viruses Explained: From Structure to Treatment

Understanding Viruses: Structure, Infection, and Immunity

A virus is a biological entity that needs a cell host to reproduce. Each virus particle, or virion, is a potentially infective agent consisting of a capsid (or protein coat) of protein surrounding the nucleic acid, which can be DNA or RNA. The shape of the capsid may be straightforward or helical, typically icosahedral (polyhedral or near-spherical), or complex, typically comprising a head and a tail. This structure can, in turn, be surrounded by the viral envelope, a lipid layer with different proteins, depending on the virus.

The life cycle of a virus always requires the metabolic machinery of the invaded cell to replicate its genetic material, producing many copies of the original virus. This process is where the destructive power of viruses lies, because they can damage or destroy the cell. They can infect eukaryotes (plants, animals, fungi, or protists) or prokaryotes (in which case they are called bacteriophages or simply phages). Some viruses require unusual enzymes, so they load them into their wrapper as part of their equipment.

Some biologists have described viruses as organisms at the edge of life, at the boundary between living matter and inert matter. Viral infections in humans and animals usually result in an immune response from the invaded organism and, often, illness or even death. Conditions include the common cold, the flu, varicella, measles, hepatitis B, yellow fever, rabies, AIDS, and so on. Often, the virus is completely eliminated by the immune system. Antibiotics, designed to combat bacteria, have no effect on viruses, but antiviral drugs have been developed for the treatment of viral infections. Vaccines can prevent viral infections by producing immunity over a long period.

Key Characteristics of Viruses

Viruses lack organelles and, above all, metabolic autonomy; therefore, they are not considered cells. Their life cycle has two phases: an extracellular phase, which is metabolically inert, and an intracellular phase, which is reproductive. The defining characteristics of viruses can be grouped around three issues: their size, the fact that they are crystallizable, and the fact that they are obligate intracellular parasites or microcellular. These three characteristics place viruses at the border between the living and the nonliving.

Small cell size: between 0.02 and 0.009 μm. Because their size is smaller than bacteria, they can pass through filters that retain bacteria.

Protozoan Parasites

Protozoa parasites are unicellular organisms characterized by a complex metabolism. They feed on solid nutrients, algae, and bacteria present in multicellular organisms, like humans and animals. They are often in the form of cysts or eggs. For example, eggs of Cryptosporidium and Giardia cysts are common in waters affected by fecal contamination.

Classes of Protozoan Parasites

They can be classified into six major groups according to the type of organisms they affect:

• Phytophagous: When they attack plants, they cause multiple diseases.
• Soófagos: When they attack animals.
• Dermatropos: Affect the skin (chickenpox, herpes, measles).
• Neurotropic: Affect the respiratory system (influenza, pneumonitis).
• Viscerotropic: Attack various viscera (viral hepatitis, etc.).
• Bacteriophages: When they attack bacterial cultures; this category is of great interest because it has allowed researchers to undertake a series of experiments that have elucidated some of the many unknowns in the field of molecular genetics.