Viruses

Wikipedia says the following about viruses.

A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus, at an extraordinary rate. Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell. Over 2,000 species of viruses have been discovered.
A virus consists of two or three parts: genes, made from either DNA or RNA, long molecules that carry the genetic information; a protein coat that protects the genes; and in some viruses, an envelope of fat that surrounds and protects them when they are not contained within a host cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses are about 100 times smaller than bacteria, it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimetre (0.39 in).
Viruses spread in many different ways. Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors. Some viruses of animals are spread by blood-sucking insects. Each species of virus relies on a particular method. Whereas viruses such as influenza are spread through the air by people when they cough or sneeze, others such as norovirus, which are transmitted by the faecal-oral route, contaminate hands, food and water. Rotavirus is often spread by direct contact with infected children. The human immunodeficiency virus, HIV, is one of several major viruses that are transmitted during sex. The origins of viruses is unclear: some may have evolved from plasmids—pieces of DNA that can move between cells, while others may have evolved from bacteria.
Viral infections often cause disease in humans and animals, however they are usually eliminated by the immune system, conferring lifetime immunity to the host for that virus. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent some viral infections.

 Scientists are actively studying various aspects of viruses.

Depressed patients not being treated with antidepressants (selective serotonin uptake inhibitors) had lower cell-mediated immunity to the varicella-zoster virus -- and were less able to respond to the shingles vaccine -- compared with patients who were not depressed or who were depressed but were receiving treatment with antidepressants, the researchers found.

An isolated outbreak of a deadly disease known as acute hemorrhagic fever, which killed two people and left one gravely ill in the Democratic Republic of Congo in the summer of 2009, was probably caused by a novel virus scientists have never seen before.Whether you become infected by some strains of rotavirus may depend on your blood type. Some strains of rotavirus find their way into the cells of the gastrointestinal tract by recognizing antigens associated with the type A blood group, a finding that represents a new paradigm in understanding how this gut pathogen infects humans, said Baylor College of Medicine researchers in an online report in the journal Nature.

"Short-term measures include risk assessment of all live virus vaccines currently registered by the APVMA in regard to the risk of recombination and could include changes to product labels, which may result in restrictions on the use of two vaccines of different origins in the one animal population."