Discovery of (1884: invention of the Chamberland filter with pores smaller than bacteria) 1892: Russian biologist Dmitri Ivanovsky publishes a paper in which shows that extracts from diseased tobacco plants remain infectious after filtration through Chamberland filter. MEANING? 1898: Dutch microbiologist Martinus Beijerinck postulates a new form of filterable infectious agent term virus introduced. By the end of the 19th century, viruses were defined in terms of their infectivity, their ability to be filtered, and their requirement for living hosts, but they still could not be directly observed! (1930: invention of the electron microscope) 1931: first images of viruses obtained using electron microscopy by German engineers Ernst Ruska and Max Knoll. Dmitri Ivanovsky
What are Viruses? A virus is a non-cellular biological particle made of genetic material (DNA or RNA) and protein coat. From the Latin vīrus referring to poison. Infect living cells to reproduce. Capable of causing diseases. Co-exist with life everywhere. About 5,000 viruses have been studied and described in detail, although there are millions of different types. The origin of viruses remains unclear because they do not form fossils
Who do viruses infect? Specific viruses usually infect a specific host. Viruses infect Bacteria These viruses are called bacteriophages. Harmless to animals/humans and can be studied easily. Lately there is growing interest in the use of bacteriophages to treat bacterial infections in humans. Viruses infect Plants One example is the tobacco mosaic virus. Often they only cause a loss of crop yield, so it is not economically viable to try to control them. Viruses infect Animals Can cause different diseases depending on the types of cell that they infect; some can cause lifelong or longterm (chronic) infections. Viral infections in animals provoke an immune response that usually eliminates the infecting virus.
Virus Structure A virus particle, called a virion, consists of: Genes nucleic acid DNA or RNA (retroviruses) But not both! Capsid a protein coat surrounding the nucleic acid Bacteriophage Capsid (Optional) Envelope some viruses are enclosed by a bubble of lipid (fat) Influenza Envelope
Virus Shapes Helical Polyhedral Spherical Complex Influenza Bacteriophage
Most viruses range in size from 20 nm to 300 nm; some are almost 1 µm! Scale Sense
Viruses are NOT Considered Living Viruses do not belong to any kingdom vs Have no nucleus, no organelles, no cytoplasm or cell membrane. Can not carry out cellular functions they are not living cells! Cannot reproduce outside the host cells: need to use the organelles and enzymes of the invaded cells.
Virus Replication Viruses can only reproduce inside the cells of living organisms: invade living cells and force them produce many thousands of identical copies of the original virus. 1. Virus attaches to a specific host cell. 5. New viruses are released as the host cell bursts open. 3. Virus genetic material is both replicated (copies made) and translated (virus proteins made). 4. New virions assemble inside of the host cell. 2. Virus genetic material enters the host cell. Some viruses do not destroy the host cell but continue to use its resources to replicate for a prolonged period of time.
S days Symptoms S S S S S S
Host Defense Innate immune system Hours Adaptive immune system Days production of specific killer T cells first-aid cells that recognize and destroy pathogens in a generic way production of specific proteins (antibodies) that bind to the virus and make it non-infectious - unable to penetrate host cells
Vaccination Vaccines were used to prevent viral infections long before the discovery of the actual viruses A vaccine against a viral disease can be made from an attenuated (weakened, less vigorous), less virulent (active/infectious) strain of the virus. Edward Jenner (smallpox vaccine, 1796 ) and Louis Pasteur (rabies, 1886) were the first to develop vaccines to protect against viral infections. Edward Jenner Attenuated virus is capable of stimulating an immune response and creating immunity, but not causing illness. Worldwide use of vaccines has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as polio, measles, mumps and rubella; smallpox have been eradicated; polio is expected to be eradicated in the nearest future. 12
Examples of Viral Diseases DISEASE Cold Wart, Cold Sores Flu VIRUS causing the illness Rhinovirus Herpes Simplex Virus Influenza Measles Morbillivirus Ebola Ebolavirus
Beneficial Role of Viruses Regulation of saltwater and freshwater ecosystems: Most of viruses are bacteriophages, and most are in the oceans, where they play a major role in regulating bacterial populations by killing ~20% of oceanic bacterial biomass each day. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms, which often kill other marine life. Viruses help maintain the ecological balance of different species of marine blue-green algae, and thus adequate oxygen production for life on Earth. Transferring genes between different species (horizontal gene transfer): It is thought that viruses played a central role in the early evolution. Numerous viral DNA sequences are scattered throughout the human genome (making up around 8% of human DNA); most of it is no longer functional, but some genes are important in human development. Similarly, viruses have transferred important genes to plants (some photosynthesis-related genes were transferred from blue-green algae).
Viruses in Ancient History First ever written record of a virus infection, 1580 1350 BC: an Egyptian stele depicts a temple priest showing typical clinical signs of paralytic poliomyelitis. Poliomyelitis (Polio) is a viral disease which may affect the spinal cord causing muscle weakness and paralysis. The disease had been a real problem in Europe and the US during late 19 th - 20 th centuries before vaccine was introduced. Year 1143 BC: preserved mummy of Pharaoh Ramses V shows lesions on his face suggesting that he died of smallpox. Smallpox is a disease caused by the Variola major virus. Some experts say that over the centuries it has killed more people than all other infectious diseases combined. Smallpox have been eradicated in the world through vaccination (last case diagnosed in 1977).
Viruses in History Year 1519: smallpox was transferred from Europe to the Americas aboard one of Hernando Cortez ships. 3,500,000 Aztecs died in the next 2 years - effectively the end of the Aztec empire. The epidemic, and those that followed during 1545 1548 and 1576 1581, eventually killed more than half of the native population. Spanish Flu, 1918-1919: unusually deadly influenza pandemic. It infected ~500 million people across the world, including remote Pacific islands and the Arctic, and killed 50 to 100 million of them three to five percent of the world's population.