A Virus is a type of pathological agent.
Unlike bacteria, which can reproduce on their own, viruses need a host cell in which to replicate. A virus penetrates the cell wall, and then uses the host cell's ability to make proteins to replicate its own RNA and protein coat. Within about half-an-hour, the host cell has created several dozen copies of the virus, which then break through the cell wall to go to infect other cells.
Because viruses can create hundreds of copies of themselves in a short time, they can spread very rapidly through a host body. Moreover, bacteria usually kill a host by releasing toxic substances in the body, while viruses usually kill a patient by destroying billions of cells within the body. In addition, since viruses are smaller and harder to kill outside the body than bacteria, they often are more contagious than bacteria. In addition, because they are much smaller, filters that will keep out bacteria will let viruses pass through.
Antibiotics have no effect on viruses. Most anti-viral drugs target a particular viral infection, although there are a few broad spectrum anti-viral drugs. However, the side effects of these drugs are usually more severe than the side effects of antibiotics.
Many less severe viruses can be treated by letting the patient rest and insuring they have sufficient fluids. When the patient is unable to take liquid due to unconsciousness or nausea, intravenous saline solution is required.
In most cases, prevention of viral contagion is preferred. Most viruses can be vaccinated against, although several, including HIV, resist all attempts to develop vaccines. However, many viral diseases that were common in the mid-20th century, including polio, measles, and rubella are almost unknown today because of vaccination.