In a healthy individual, the majority of the body's tissues are sterile. The inner tissues, the blood and the interstitial fluids are all free of any cells that are not inherent to the human body. Nevertheless, the different layers of the skin, the mucosal surfaces and most of all the gastrointestinal tract (especially the intestines) are home to a wide variety of microorganisms, mostly bacteria but also some fungi and unicellular organisms that inhabit our body. These microorganisms are collectively referred to as the natural human flora or microbiota and they exist in all healthy people. In immune-deficient individuals, or in cases of trauma to the inner tissues, the flora can expand and even migrate to normally non-penetrable tissues, leading to sickness. In contrast, in a healthy individual the natural flora competes with potentially disease-perpetrating (pathogenic) microbes and thus limits their infective abilities. Additionally, members of the gut microbiota are very helpful in digesting different nutrients in our food, so all in all the natural flora is beneficial to human health.
E. Coli (Escherichia coli) bacteria are the most widely-studied members of the gut microbiota. Right: as observed under a scanning electron microscope (adopted from Wikipedia). Left: fluorescent microscope image (red, green and yellow mark various parts of the bacterial envelope, while blue is bacterial DNA).
It is currently estimated that on average each individual is made up of 1013 human cells. The human microbiota consists of several thousand species, and is estimated to be made up of 1014 microbial cells – 10 times the number of human cells!. It is important to stress that the number of cells in the human body has never been accurately counted, and these numbers are only rough estimates. In addition, the number of cells and bacteria changes between individuals, as well as with time. Most of the microbiota is comprised of bacteria that are much smaller than human cells: 10,000-100,000 bacterial cells can fit within an average human cell. Due to these great differences in size, 1014 bacteria would occupy 0.01% of the volume taken up by 1013 human cells. Since the human body also includes a significant amount of extracellular mass, we can conclude that the microbiota constitutes an even smaller percentage of the total body mass.
The dry mass of the human microbiota sums up to a few grams, similar to some human body organs. In this respect, due to the importance of the flora to human health, some scientists refer to it as "the forgotten organ". In contrast to most human cells in the body, the microbiota is characterized by an enormous rate of renewal, as evident from the fact that 60% of the dry mass in human feces is attributable to bacteria. This means that much of the gut microbiota is excreted and renewed every few days.
Until now I discussed the part of the human microbiota that is mostly composed of bacteria, as well as some fungi and other unicellular organisms. In your question, however, you were also concerned with the number of viruses. Since most of the human body is protected from the external environment, most viruses in a healthy human are found on the skin, in mucosal tissues and in the digestive system. The vast majority of these viruses are unknown and there is no quantitative data about them. One of the reasons for this is that most of these viruses have adapted to enter or attack our microbiota rather than our own cells. The scientific field of virology mainly deals with a small number of human-infecting viruses that are associated with disease, such as influenza, hepatitis, herpes and AIDS. Since, as mentioned, most of the viruses in our body are found inside cells of the microbiota, it can be postulated that these viruses also have a significant effect in regulating the size and diversity of the human microbiota.
To summarize, our body serves as a habitat that is constitutively inhabited by a rich and complex microbiota. Despite its tiny mass, this community of microbes outnumbers the human cells by a factor of ten. Even though these microbial populations are constantly renewed, the microbiota is maintained at a steady state that is the product of the interrelationships between the different members of the microbiota and between the microbiota and the human body.