Viruses are the smallest organism. There are many living organisms that are very small but non-living organisms are a lot smaller. Alot of biologists consider viruses to be non-living because they lack a cellular structure and cannot metabolize by themselves. They need a host cell to replicate and survive. Because viruses have genetic material and can literally eat their host, they are considered organisms. The smallest viruses in terms of genome size are single-stranded DNA viruses. The most famous one has 5386 nucleotides. However, some viruses can be even smaller. Some can be even as small as 2000 nucleotides.