Question

why disease caused by virus are more dangerous explain

Answer 1

Many human infections are caused by either bacteria or viruses. Bacteria are tiny single-celled organisms, thought by some researchers to be related to plants. They are among the most successful life forms on the planet, and range in habitat from ice slopes to deserts. 

Bacteria can be beneficial – for instance, gut bacteria help us to digest food – but some are responsible for a range of infections. These disease-causing varieties are called pathogenic bacteria. Many bacterial infections can be treated successfully with appropriate antibiotics, although antibiotic-resistant strains are beginning to emerge. Immunisation is available to prevent many important bacterial diseases. 

A virus is an even smaller micro-organism that can only reproduce inside a host’s living cell. It is very difficult to kill a virus. That’s why some of the most serious communicable diseases known to medical science are viral in origin.

Answer 2

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To understand viruses, it may help to consider the French emperor Napoleon Bonaparte. In the early 19th century, Bonaparte invaded much of Europe in order to establish French dominance over the continent. He's also known for being somewhat short in stature (however unfair that reputation may be).

Like our idea of Napoleon, viruses are very small -- 100 times smaller than the average bacterium, so small that they can't be seen with an ordinary microscope. Viruses can only exert influence by invading a cell, because they're not cellular structures. They lack the ability to replicate on their own, so viruses are merely tiny packets of DNA or RNA genes enfolded in a protein coating, on the hunt for a cell they can dominate.

Viruses can infect every living thing -- from plants and animals down to the smallest bacterium. For this reason, they always have the potential to be dangerous to human life. Still, they don't become truly treacherous until they infect a cell within the body. This infection can happen several ways: by air (thanks to coughing and sneezing), via carrier insects like mosquitoes, or by transmission of body fluids such as saliva, blood or semen.
Once a virus infects a cell, it tries to take over its host completely, much as Napoleon spread the French influence with every country he fought. A virus lodged in a cell replicates and reproduces as much as possible; with each new replication, the host cell produces more viral material than it does normal genetic material. Left unchecked, the virus will cause the death of the host cell. Viruses will also spread to nearby cells and begin the process again.

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