* Intracellular Parasites: Viruses are obligate intracellular parasites, meaning they absolutely need a host cell to survive and replicate.
* Lack of Independent Replication: Viruses lack the machinery to reproduce on their own. They rely on the host cell's ribosomes, enzymes, and energy sources to create new viral particles.
* Viral Replication Cycle: Once inside a host cell, a virus will hijack the cell's machinery to replicate its own genetic material (DNA or RNA) and proteins. These components then assemble into new viral particles, which can then infect other cells.
Examples of viruses that infect human cells:
* Influenza virus: Causes the flu.
* HIV (Human Immunodeficiency Virus): Causes AIDS.
* Herpes simplex virus: Causes cold sores and genital herpes.
* Hepatitis B virus: Causes liver disease.
* COVID-19 virus (SARS-CoV-2): Causes the disease COVID-19.
Important Note: While bacteria can cause infections, they don't usually enter host cells to replicate. They typically live outside of cells and can produce toxins that cause disease.
