virus is a microscopic infectious agent that can reproduce only inside a host cell. Viruses infect all types of organisms: from animals and plants, to bacteria and archaea. Since the initial discovery of tobacco mosaic virus by Martinus Beijerinck in 1898; more than 5,000 types of virus have been describing in detail, although most types of virus remain undiscovered. Viruses are ubiquitous, finging in almost every ecosystem on Earth; and are the most abundant type of biological entity on the planet.

Viruses such as hepatitis B can cause life-long or chronic infections; and the viruses continue to replicate in the body despite the hosts’ defence mechanisms. In some cases, these chronic infections might be beneficial as they might increase the immune system’s response; against infection by other pathogens. However, in most cases viral infections in animals cause an immune response that eliminates the infecting virus.

Immune responses produced by vaccines

These immune responses can also be producing vaccines that give lifelong immunity to a viral infection. Microorganisms such as bacteria also have defences against viral infection; such as restriction modification systems. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat both life-threatening and more minor infections.

US health authorities have given their approval to dengue vaccine Dengvaxia; the controversial first treatment designed to protect against the deadly mosquito-borne virus. The Food and Drug Administration made the announcement on Wednesday; but the use of the vaccine can only occur under certain strict conditions: only children aged between nine and 16; who have previously confirmed infections, and who live in endemic areas, may receive the dose.

“While there is no cure for dengue disease approval is an important step; toward helping to reduce the impact of this virus,” said Anna Abram; a senior FDA official. The virus is endemic in the US territories of American Samoa, Guam; Puerto Rico and the US Virgin Islands, said the FDA.

Mosquito-borne virus

Dengue, which causes hemorrhagic fever, is the world’s most common mosquito-borne virus; and infects an estimated 390 million people in more than 120 countries each year; killing more than 25,000 of them, according to the World Health Organization.

But Dengvaxia, produced by the French pharmaceutical firm Sanofi Pasteur; is controversial: the Philippines, which became the first country to approve it in 2016; was also the first to ban it in February this year over safety concerns. Manila is also pursuing criminal charges against the company’s officials; over the deaths of children injected with Dengvaxia.

Sanofi denies the charges but in 2017 admitted the vaccine was not suitable for people; who had not previously been infected with the virus. Dengvaxia has previously been approved in 19 countries and the European Union; which greenlit the vaccine under similar conditions to the US, though the age range was nine to 45-years-old.