Viruses are the smallest known parasitic intracellular particles.
Viruses can consist of an envelope, capsid (protein shell encasing the genetic material), genome (RNA or DNA), and non-structural viral proteins.
Their replication is specifically dependent on the host cell, whether it be a bacteria, plant, or animal. The nucleic acid core of the virus houses the genetic material, which it releases into host cells to initiate duplication and ultimately infection.
They are invisible to light microscopy, ranging from 20-250nm, approximately 40 times smaller than the average bacterium. Hundreds of different viruses are known to infect humans, as well as other living things.
Viruses are often the cause of contagion and have resulted in worldwide disease outbreaks. Some recent examples were the outbreak of Ebola in West Africa in 2014, the 2009 H1N1 / Swine Flu, and currently the COVID-19 pandemic.
sources & references:↓
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30183-5/fulltext
https://www.elsevier.com/__data/assets/pdf_file/0010/977698/novel-coronavirus-covid-19-infection-2020-04-28.pdfMerck Manual of Diagnosis and Therapy. 16th ed., Rahway, Merck, 1992.
Volk, Wesley A., and Margaret F. Wheeler. Basic Microbiology. 5th ed., Cambridge [etc.], Harper & Row, 1984.
image source: https://www.cdc.gov