No. Most bugs, or microbes, live in and on our bodies without ever causing us to be ill. In fact, there are more bacteria in the healthy human gut than there have ever been people on earth. These bacteria and other bugs in the gut are known as the microflora. This microflora helps us to: digest our food; breakdown drugs and chemicals such as hormones; produce some important vitamins; and protects against invasion by harmful microbes. Only a minority of bugs make us ill.
Occasionally, infection can be caused by a bug that normally lives within our body or in the environment around us without causing us any problems. This happens when our immune system is weakened, perhaps because of another disease such as AIDS, or when we are having chemotherapy or radiotherapy. These infections are called opportunitstic infections because the bug takes advantage of an opportunity to cause disease. Some groups of people are more susceptible to these infections than others, e.g. the elderly, babies and children.