Viruses

Viruses are one of the most common submicroscopic infectious agent in the world found in almost every ecosystem. Viruses only replicates inside the cell of a living organism. 

Viruses not only cause harm and death to humans, they also infect plants and animals. Viruses are hard to cure and so far many have no cures because they are surrounded by protective protein coating and they do not have cell wall that can be attacked by antibodies. 

Viruse like the HIV are one of the world's deadliest because this type of virus has a way of  hidding or tricking the immune system by presenting multiple conformation  of HIV envelope protein to the body's cells. This leads the immune system into mounting a response that is ineffective.

Below is a picture of a virus