Viruses are not alive because they do not complete all of the seven life processes:
We say 'strains' of virus and not species. Most are made of a relatively short length of the genetic material DNA which is surrounded by a protein coat.
Viruses can often survive outside a host for long periods of time. When they have infected a suitable host cell or cells, they replicate themselves within the cell thousands of times. They do not divide and reproduce, but replicate their DNA and protein coats. These are then assembled into new virus particles. The host cell or cells then burst and other nearby cells can be infected with the virus. This process can be as quick as twelve hours in the case of the Norovirus or several days for Ebola.
Viral infections cannot be treated by antibiotics.
The tobacco mosaic virus infects tobacco and lots of other closely related species, such as tomatoes and peppers. It is transmitted by contact between plants, either naturally or through the hands of farmers. It infects the chloroplasts of plant leaves and changes their colour from green to yellow or white in a mosaic pattern. It can also make leaves crinkle or curl up.
There is no cure, therefore farmers must try to reduce the spread of the virus.
HIV stands for Human Immunodeficiency Virus. This infection is transmitted by body fluids, often during unprotected sex, but also through cuts and injecting drugs using shared needles. Immediately after infection, people often suffer mild flu-like symptoms. These pass and for a period of time infected people might not know they are infected.
AIDS stands for Acquired Immune Deficiency Syndrome. Months or years after the infection of the HIV virus, it becomes active and starts to attack the person's immune system. HIV at this point has become AIDS.
HIV is detected by screening patients for antibodies against proteins that are found on the virus. There is no cure for HIV /AIDS although many scientists are trying to find one. Currently, infected people are given antiviral drugs, which can slow the development of AIDS.
Measles is a very infectious viral disease that is often caught by young children. It is transmitted through the air in tiny droplets after an infected person sneezes. It causes a fever and skin rash. Many children in developed countries are given vaccines against measles, but this is not the case throughout the world. Infection can lead to serious and potentially life-threatening complications in some people.