Viruses are the smallest infectious agents that can affect humans, animals, plants, and also bacteria and fungi. Each virus has its "own host" and can only infect specific cells.
The main characteristic of viruses is that they are not fully-fledged living organisms. They don't have a cellular structure or their own mechanisms for reproduction. Instead, a virus penetrates the body's cells and uses their "machinery" to create new copies.
Structurally, a virus consists of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. Some viruses have an additional outer envelope called a "supercapsid".
How Do Viruses Enter the Body?
Infections are most often transmitted through mucous membranes—the nose, mouth, eyes, genital organs, and anus. Infection can also occur through damaged skin or insect bites (mosquitoes, ticks).
How Do Viruses Work?
Once in the body, a virus attaches to a cell and penetrates it. The subsequent path can vary:
•
Lytic cycle
— the virus immediately begins to actively multiply, destroying the cell.
•
Lysogenic cycle
— the virus "falls asleep" inside the cell, integrating into its genetic code. Under certain conditions (stress, weakened immunity), it activates and begins to spread.
Main Types of Viruses
There are a huge number of viruses, among which the most well-known are:
• Influenza (Influenza A and B) — causes seasonal epidemics.
• Herpesviruses (Herpesviridae) — cause herpes, chickenpox, and shingles.
• Coronaviruses — including SARS-CoV-2 (COVID-19).
• Human Papillomavirus (HPV) — causes warts and can be linked to oncological diseases.
• Enteroviruses — including poliomyelitis and hand, foot, and mouth disease.
• Flaviviruses — transmitted by mosquitoes (dengue, yellow fever, Zika virus).
• Hepatitis A, B, C — affect the liver and can lead to chronic diseases.
• HIV and retroviruses — integrate into human DNA, causing severe chronic illnesses.
Diseases Caused by Viruses
Clinical manifestations depend on the specific pathogen. Viral infections can be asymptomatic but can also cause severe diseases:
• colds and flu
• COVID-19
• measles, chickenpox
• hepatitis
• HIV/AIDS
• papillomavirus infections
• herpes
• poliomyelitis
• rabies
Living or Non-living?
Scientists still debate whether to consider viruses as living organisms. On one hand, they don't have a metabolism and cannot exist without a host. On the other hand, they are capable of reproduction and change (evolution). Therefore, viruses occupy a "grey zone" between living and non-living.
Why Is It Important to See a Doctor?
Many viral infections are mild and resolve on their own. However, a number of diseases can have serious consequences: chronic liver damage, oncological processes, and complications to the respiratory system and immunity. Only a doctor can make an accurate diagnosis and prescribe treatment or prevention (for example, vaccination).