VIRUSES

• 1897: a Dutch scientist suggested that something smaller than a bacteria could cause disease.

• 1935: Wendell Stanley isolated crystals of tobacco mosaic virus. Reasoned that living organisms do not crystallize, so he inferred that viruses were not truly alive.

THE DISCOVERY OF VIRUSES

VIRUSES CAN REPRODUCE ONLY BY INFECTING LIVING CELLS.

Structure and composition• The protein coat

surrounding a virus is called a capsid.

• Most viruses have proteins on their surface membrane or capsid that bind to receptor protein on the cell

• Viruses must bind precisely to proteins on the host cell and then use the host’s genetic system.

• Most viruses can only infect a very specific kind of cell.

• Viruses that infect bacteria are called bacteriophages.

VIRAL INFECTIONS

• Inside living cells, viruses use their genetic information to make multiple copies of themselves. Some viruses replicate immediately, while other initially persist in an inactive state within the host.

WHAT HAPPENS AFTER A VIRUS INFECTS A CELL?

LYTIC INFECTIONS

Lysogenic Infections

RNA VIRUSES• About 70% of viruses contain RNA rather than DNA.

• Example: Cold viruses—invade a host cell, make many copies of the viral RNA. Host cell makes other viral proteins and capsids.

• Example: HIV– called a retrovirus. When virus invades host cell, makes a DNA copy of its RNA which inserts into the host’s DNA. This DNA may remain very inactive for many years.

• Caused by new or reappearing infections agents that typically exist in animal populations.

• Ex. Ebola—animals living in tropical forests expose humans clearing forests for homes or agriculture.

• Ex. Hantavirus, virus is found in deer mice and other rodents, causes a very dangerous form of pneumonia in humans.

• Ex. SARS –may have transferred to humans from civet cats.

• Ex. Avian flu---what is the current situation?

EMERGING VIRAL DISEASES

• Viral disease is prevented and treated through vaccination, vector control and drug therapy

• Vaccinations: a harmless versions of the virus, bacterium or a toxin causes an immune response.

• Viral vaccines can be made from inactivated (can’t replicated in a host), or attenuated (weakened virus can’t cause disease)

PREVENTION AND TREATMENT

• Vector control: control animals that carry the virus, like mosquitos for yellow-fever, or rabies vaccinations for pets to prevent humans from getting rabies.

• Drug Therapy: Some antiviral drugs interfere with viral nucleic acid synthesis.

• Difficult to design a drug that blocks the virus but doesn’t harm cells.