A virus is a microscopic infectious agent that requires a host cell to survive and replicate. Viruses are not considered living organisms because they cannot reproduce or carry out metabolic functions on their own. Instead, they invade host cells and use their machinery to make more copies of themselves.
Key Characteristics of Viruses
🔹 Acellular (Not Made of Cells) – Unlike bacteria or fungi, viruses are simply genetic material (DNA or RNA) enclosed in a protein coat.
🔹 Require a Host Cell – Viruses cannot reproduce on their own; they must infect a living cell to multiply.
🔹 Small in Size – Typically 20-300 nanometers, much smaller than bacteria.
🔹 Genetic Material Can Be DNA or RNA – Some viruses store their genetic instructions in DNA (like herpesvirus), while others use RNA (like influenza or HIV).
🔹 Mutate Rapidly – Many viruses evolve quickly, leading to new strains (e.g., flu viruses, COVID-19 variants).
Structure of a Virus
| Component | Function |
|---|---|
| Capsid (Protein Coat) | Protects the virus’s genetic material |
| Genetic Material (DNA or RNA) | Carries the instructions for replication |
| Envelope (Some Viruses) | A lipid layer that helps some viruses enter host cells |
| Spike Proteins | Help the virus attach and enter host cells (e.g., coronaviruses) |
💡 Viruses with an envelope (like influenza and HIV) are often more fragile outside the body than non-enveloped viruses (like norovirus and poliovirus).
How Viruses Infect Cells (Replication Process)
1️⃣ Attachment – The virus binds to a receptor on the host cell’s surface.
2️⃣ Entry – The virus or its genetic material enters the cell.
3️⃣ Replication & Assembly – The virus hijacks the host cell’s machinery to produce viral components.
4️⃣ Release – New virus particles burst out (lysis) or bud off to infect other cells.
💡 This process is why viruses spread rapidly and cause infections!
Types of Viruses & Examples
🦠 DNA Viruses (More Stable, Mutate Slowly)
- Herpesvirus (cold sores, genital herpes)
- Hepatitis B virus (HBV)
- Human papillomavirus (HPV) (linked to cervical cancer)
🦠 RNA Viruses (Mutate Quickly, More Unstable)
- Influenza virus (flu)
- Coronavirus (SARS-CoV-2, COVID-19)
- HIV (Human Immunodeficiency Virus)
- Rabies virus
🦠 Retroviruses (Convert RNA into DNA using reverse transcriptase)
- HIV/AIDS
How Viruses Spread
🔹 Respiratory Droplets – Coughing, sneezing (e.g., COVID-19, flu)
🔹 Direct Contact – Skin-to-skin, bodily fluids (e.g., herpes, HIV)
🔹 Contaminated Surfaces – Touching infected surfaces (e.g., norovirus)
🔹 Vector-Borne Transmission – Spread by insects (e.g., Zika virus, dengue, West Nile)
💡 Some viruses can survive on surfaces for hours to days, while others break down quickly outside a host!
Immune Response to Viruses
1️⃣ Innate Immunity (First Line of Defense)
- Natural Killer (NK) cells attack virus-infected cells.
- Interferons slow down viral replication.
2️⃣ Adaptive Immunity (Targeted Response)
- T-Cells (CD8+) destroy infected cells.
- B-Cells produce antibodies that neutralize the virus.
💉 Vaccines train the immune system to recognize viruses before infection occurs (e.g., flu vaccine, COVID-19 vaccines).
How to Protect Against Viruses
✔ Vaccination – Prevents many viral infections (e.g., measles, flu, HPV).
✔ Good Hygiene – Handwashing, mask-wearing, and surface cleaning reduce transmission.
✔ Strong Immune System – A healthy diet, exercise, and sleep help fight infections.
✔ Antiviral Medications – Some viruses (e.g., HIV, flu) can be treated with antivirals, but antibiotics do NOT work against viruses.
Final Thoughts
Viruses are tiny infectious agents that require a host cell to survive and multiply. They cause a wide range of diseases, but vaccines, hygiene, and a strong immune system help protect against them. Unlike bacteria, viruses do not respond to antibiotics, making prevention and immunity the best defense.