Our most notable encounters with microbes are probably those that lead to disease. We should be aware of the distinction between infection and disease, because we use the terms interchangeably and incorrectly. Infection refers to the invasion of healthy tissues by microbes capable of causing disease. Presence of the microbes. Disease occurs when the body has a measurable response to their presence. That is, their presence causes a cough or a fever or a rash or an antibody titer or feeling of malaise...things we call signs and symptoms. Reaction to the microbes. So though we may be infected with potential pathogens, we don't cross over into disease until the body responds.
That said, we should first consider that there are microbes covering our body surfaces at all times. These constitute the "normal flora" or "normal microbiota" of generally harmless microbes that make up 90% of the cells in the space you occupy (Yes, your cells are only 10% of all cells present in and on your body! In fact, every time you defecate you lose more cells than you retain because of the microbial content of your gut!). They protect us by inhabiting the niches of our skin and mucous membranes surfaces to prevent colonization by potential pathogens. They populate out digestive tract and form plaque on teeth. They are the normal residents of those niches. However, just as tourists pass through our cities and interact with citizens and landmarks, "transient microbes" that may or may not pose a potential threat to health encounter the niches of our bodies from time-to-time. The results vary with the strengths and abilities of the "visitor", and some possess unique traits that allow them to disrupt normal life for microbes and the niche affected. Your textbook will provide added coverage of these topics and the general concepts of health and disease in greater detail.
Illness is inevitable in each of us. Sometimes the pathogenic microbe is transmitted from one sick individual to a healthy one, and in doing so disease is spread. There are a variety of factors and mechanisms by which disease is spread, including the mode of transmission, the portal of entry into the body, and whether a vector or fomite is involved. Some diseases are of particular interest to the CDC and must be reported in numbers and locations when found in a community. And it becomes more crucial when there is a localized outbreak beyond expectations in numbers or severity, something called an epidemic. When such an outbreak occurs worldwide, it is called a pandemic. Ability to identify the causative agent of an epidemic or pandemic and to trace its spread among the population is a key factor toward controlling dangerous outbreaks around the world.
All of these terms and concepts are discussed in greater detail in your textbook. And in reading, you will find that there are some diseases that are of such concern to health officials that they must be reported to the CDC for national monitoring purposes. So, the concept of public health and microbial diseases has many dimensions. Entire courses are devoted to the topics we are glossing over in this introduction to the topic. In this section, we will provide an introduction to some basic concepts and terms used in the field.