Fever, fatigue and a weakened immune system that could lead to more diseases, and worse, death are common features of viral infections. Influenza, measles, HIV, and just most recently, Ebola are among the commonly known and much feared diseases that are caused by different types and strains of viruses. However, not all viruses are the same; not all could lead to death as well. A recent study revealed that a certain virus could kill-- not lives -- but brain cells, making the infected individual "stupid".

The culprit: chlorovirus ATCV-1, a virus only known to appear in freshwater algae.

The study which involved 92 participants found that the chlorovirus ATCV-1 was present in 40 of the volunteers. The researchers also discovered that those with the virus performed measurably worse in cognitive testing. These infected were said to have performed worse in intelligence tests, even when education and age were taken into account.

Study author Professor Robert Yolken of Johns Hopkins medical school said that for the first time, scientists are investigating  millions of viruses living in the human body.

"We're really just starting to find out what some of these agents that we're carrying around might actually do," he said. "It's the beginning, I think, of another way of looking at infectious agents -- not agents that come in and do a lot of damage and then leave, like Ebola virus or influenza virus. This is kind of the other end of the spectrum. These are agents that we carry around for a long time and that may have subtle effects on our cognition and behavior," Yolken explained.

The research suggests that the ATCV-1 virus alters the genes in the brain.

In tests with mice infected by the virus, there was a seen decrease in recognition memory and other brain function of the mice. The virus altered the activity of genes in the animal's brains. Also, the genes affected including those producing dopamine - a vital hormone which influences memory, spatial awareness, emotion and pleasure.

Professor James Van Etten, a biologist from the University of Nebraska said, "There's more and more studies showing that microorganisms in your body have a bigger influence than anything anyone would have predicted, and this could be something along those lines."

The Johns Hopkins University and the University of Nebraska-led research was published in the Proceedings of the National Academy of Sciences.