People entering the U.S. first need to get a visa. A visa allows the visa holder to travel to a port of entry in the United States and request the immigration officials permission to enter the country. The visa could be a non-immigrant visa if the person intends to stay in the U.S. temporarily.
Tobacco mosaic virus (TMV) is a positive-sense single stranded RNA virus that infects a wide range of plants, especially tobacco and other members of the family Solanaceae.The infection causes characteristic patterns, such as  mosaic -like mottling and discoloration on the leaves (hence the name).TMV was the first virus ever to be discovered.Although it was known from the late 19th century that an infectious disease was damaging tobacco crops, it was not until 1930 that the infectious agent was determined to be a virus.lthough it was known from the late 19th century that an infectious disease was damaging tobacco crops, it was not until 1930 that the infectious agent was determined to be a virus.
1 Researchers from Europe estimate that some time in the early 1930s a form of simian immunodeficiency virus, SIV, was transmitted to humans in central Africa. 2  The mutated virus was later identified as the first of other human immunodeficiency viruses, HIV-1. A San Francisco prostitute gives birth to the first of three children who were later diagnosed with AIDS. 2  The children's blood was tested after their deaths and revealed an HIV infection. 3  The mother died of AIDS in May 1987.