How does viruses cause disease

Within these categories are different types of viruses. A coronavirus, for example, has a sphere-like shape and a helical capsid containing RNA. It also has an envelope with crown-like spikes on its surface. Seven coronaviruses can affect humans, but each one can change or mutate, producing many variants. Learn more about coronaviruses here. Just as there are friendly bacteria in the intestines that are essential to gut health , humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli.

Viruses do not leave fossil remains, so they are difficult to trace through time. Scientists use molecular techniques to compare the DNA and RNA of viruses and find out more about where they come from.

Three competing theories try to explain the origin of viruses. In reality, viruses may have evolved in any of these ways. The regressive, or reduction, hypothesis suggests that viruses started as independent biological entities that became parasites.

Over time, they shed genes that did not help them parasitize, and became entirely dependent on the cells they inhabit. In this way, they gained the ability to become independent and move between cells.

The virus-first hypothesis suggests that viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells on Earth appeared, billions of years ago.

When a viral disease emerges, it is not always clear where it comes from. A virus exists only to reproduce. When it reproduces, particles spread to new cells and new hosts. The features of a virus affect its ability to spread. Some viruses can remain active on an object for some time. If a person with the virus on their hands touches an item, the next person can pick up that virus by touching the same object.

The object is known as a fomite. Viruses often change over time. Some of these changes are very small and do not cause concern, but others can be more significant.

Significant changes could make a virus more transmissible, as has been the case with the B. They may also help the virus evade the immune system or existing treatments. For example, doctors use several drugs in combination to treat HIV so that it is harder for the virus to develop resistance to treatment.

Influenza viruses can also do so-called antigenic shift. This can happen if a host cell has become infected with two different types of influenza virus. For instance, pigs can often serve as a mixing vessel for avian and human influenza viruses. One explanation for viral virulence is that it facilitates transmission.

However, a comparison of infections caused by two enteric viruses, poliovirus and norovirus, does not support this general view. Both viruses infect the gastrointestinal tract and are spread efficiently among humans by fecal contamination. However, norovirus infection causes vomiting and diarrhea, while poliovirus infection of the intestine is without symptoms the rare invasion of the nervous system, and subsequent paralysis, is an accidental dead end.

Both viruses have successfully colonized humans for many years, so why does only one of them cause gastrointestinal tract disease? Two recent studies of bacterial virulence provide some clues about the evolution of virulence.

In one a commensal strain of Escherichia coli was serially propagated in the presence of macrophages, which are cells of the immune system that take up and destroy the bacteria. After many such passages, bacterial clones were isolated that escape phagocytosis and killing by macrophages.

These clones had also acquired increased pathogenicity in mice. Prevention and Risk Factors. Related Issues. Clinical Trials. Article: Protective efficient comparisons among all kinds of respirators and masks for Article: Distinct mechanisms govern populations of myeloid-derived suppressor cells in chronic viral Viral Infections -- see more articles.

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