Understanding Viruses: Structure, Classification, Replication, and Diseases
Structure and Composition of Viruses
Viruses are considered agents on the frontier of life. They are not cells, lack independent movement and feeding, and cannot reproduce on their own. They are smaller than bacteria and are considered distinct from any living being. They are a unique kingdom, separate from other kingdoms like bacteria.
A virus’s structure consists of two main components:
- Capsid: A protein structure made up of small pieces called capsomeres. This protects the genetic material.
- Genetic Material: This can be either DNA or RNA.
The capsid sometimes has receptors that allow the virus to attach to the host cell it infects. Some viruses also have a membrane derived from the host cell surrounding the capsid.
Classification of Viruses
We will discuss three types of viruses:
- Helical Viruses: These are elongated viruses where the capsomeres are arranged in a helix, forming a tubular structure. The DNA or RNA is located inside.
- Icosahedral Viruses: These viruses have 20 triangular faces, each composed of five capsomeres. The genetic material is always inside.
- Bacteriophage Viruses: Also known as complex viruses, these parasitize bacteria. They consist of a capsid with two parts: an icosahedral head and a tubular tail adapted for infection. The tail has spikes and fibers at the base that help the virus attach to the bacteria it infects.
Viral Replication
Since viruses are not living cells, they need a host cell to replicate and maintain their continuity. A virus enters a cell in various ways, and two things can happen:
- Multiplication (Replication): The virus’s capsid disassembles, releasing the genetic material into the host cell. The viral genetic material takes over the cell’s machinery, producing new viral components. These components assemble into new viruses, which then exit the cell, either killing it or taking part of its membrane.
- Latency: The virus’s genetic material can remain dormant within the host cell. Sometimes, the viral genetic material integrates into the host cell’s DNA, remaining inactive until triggered to replicate.
Viral Diseases
Influenza
Influenza is a viral respiratory disease that commonly appears as epidemics during winter. It has a polyhedral shape and contains RNA, spreading through the air.
Symptoms: Fever (rarely exceeding 39-40 degrees Celsius), headache, mucus, and severe coughing. Symptoms usually disappear after 2-5 days, but fatigue may persist for weeks.
Treatment: Focuses on controlling symptoms with analgesics, hydration, etc. Antibiotics are only used for bacterial complications.
Prevention: Vaccination is effective if the virus does not mutate.
Influenza A
Influenza A is similar to the common flu but its genetic material is a recombination of human, avian, and swine influenza viruses.
Symptoms: More intense muscle pain, high fever at the onset, and a more severe cough compared to the common cold.
Treatment: Antiviral or retroviral medications.
Prevention: Frequent handwashing, room ventilation, and using disposable materials.
Hepatitis
Hepatitis is inflammation of the liver tissue. Different viruses can cause various types of hepatitis, the most common being A, B, and C.
Contagion:
- Hepatitis A: Spreads through contact with food or water contaminated with the virus.
- Hepatitis B: Spreads through contaminated blood transfusions, shared needles, and sexual contact. It can also be transmitted from mother to child through the placenta.
- Hepatitis C: The mode of transmission is not fully understood but is believed to involve contaminated blood.
Symptoms: Typically begins with general malaise, weight loss, nausea, fever, and rash. After a few days, the skin and mucous membranes may develop a yellowish color (jaundice).
Treatment:
- Hepatitis A: Usually resolves without complications after a few weeks, although fatigue may last for months.
- Hepatitis B: Involves symptom control and medical monitoring, as it can become chronic and potentially lead to cirrhosis.
- Hepatitis C: More complex and requires hospital monitoring. It is often chronic.
Prevention: Vaccination is effective for Hepatitis A and B. Avoiding contact with infected individuals is also important.
AIDS (Acquired Immune Deficiency Syndrome)
AIDS is characterized by a weakened immune system that fails to defend against infectious agents.
In 1986, the Human Immunodeficiency Virus (HIV) was identified as the cause of AIDS.
Contagion: HIV spreads through blood, sexual contact, and from mother to child during pregnancy.
Symptoms: Initial infection may not produce noticeable symptoms. After a few months, the body develops antibodies to the virus, which can be detected through testing. Infected individuals may remain asymptomatic for years.
As the disease progresses, it is characterized by recurrent infections caused by opportunistic pathogens. Another symptom is the development of Kaposi’s sarcoma, a type of cancer that appears as pink patches on the upper body. Neurological disorders may also occur.
Treatment: While there is no cure for AIDS, treatments are available to manage the symptoms and slow the progression of the disease.