A feature that is unique to an infectious disease, is that you can potentially share it with your friends and family and sometimes strangers. But infectious disease has two components, there's an infection and there is a disease. And we'll see not all disease is caused by an infection. Not all infections cause disease. Not all infections can be shared. And not everyone is equally likely to become infected. Let's look at these topics one by one. What is disease? There is a universal caricature of a sick person or animal, usually showing fevers and chills, aches, fatigue. These are examples of generalized symptoms of disease, which most people have experienced. Disease results, when the ability of the cell or tissue to perform a normal physiological process is compromised. This is often due to damage or loss of function of key molecules or cells and leads to the visible features that we term symptoms. As an example, a cough is a symptom, that results from some irritation to the cells in the airways. Pathology is the discipline focussed on the study of disease and aims to link disease symptoms with the underlying molecular and cellular causes. Not all disease is due to infection. For example, genes that you inherit from your parents can have an altered function and predispose you to disease. Chemicals in the environment such as lead, can also cause disease. The symptoms for disease caused by infections, chemicals or genetics can be similar, so, it's important to know which is responsible for the symptoms to provide appropriate treatment and prevention strategies. What is infection? There are numerous organisms that consider us hospitable for sustaining their life style. Infection is a process by which one organism often a micro-organism replicates and leaves progeny in another organism, such as an animal; either on surfaces such as skin, in our guts or in any of the many cells that we have. These not us organisms can enter, derive energy and nutrients, reproduce, often by borrowing the machinery to do so from us and produce sometimes exceedingly large number of progeny. Some infections are transient interlopers, while others are like visitors who setup permanent residence like any guest, they can be largely unnoticed, and might even be helpful. The ones we are most likely to notice are those who wreak immediate havoc. These are the agents that we associate with disease, we call them pathogens and are most common in epidemics. Infection then refers to the replication of organisms in or on our bodies that might or might not cause disease; and as a note, although many types of organisms can infect other organisms, viruses can only replicate when inside the cells of an individual. They can survive in the environment, but can't replicate without the support of the host. Infection, the replication of a foreign organism, is not the same as disease, the clinical symptoms experience by the host organism. If an infection causes disease we call it a symptomatic infection, if there is no disease it is termed an asymptomatic infection. What is infectious? Well, let's use an example of a real infection that everyone knows. Influenza virus causes the disease influenza, we usually just call it flu. When you become infected with this virus, it will start replicating itself in your nose and throat. When the number of progeny in your nose increase you can transmit it to someone in close contact when you sneeze or cough. You are infectious or more commonly referred to as contagious when you are able to give the progeny of the influenza virus that infected you to someone else. Transmission is a critical step in causing epidemics. Without it, the number of people who are infected won't increase. Because you can be infected but feel perfectly well, this would be an asymptomatic infection, you could be unwittingly spreading the infection to others. However, with the many micro-organisms that do cause disease there's a correlation between having symptoms of disease and being infectious. This makes sense when a virus infects you and starts to reproduce, it's using resources that you might need so you will have some symptoms of disease such as coughing and sneezing. The virus must produce sufficient numbers of progeny to optimize transfer and infection of the new host, this is why replication of the virus, disease symptoms and being infectious can be related. The length of time you are capable of transmitting the virus to someone else is called the infectious period, which is an important factor in studying the properties of epidemics. So, are some people more likely to get infected than others? You probably noticed that some people never get sick when everyone else does. Infectious agents need a hospitable environment to replicate for example, if you don't have all the resources the agent needs, it will not be able to efficiently reproduce in you. Similarly, you can have plenty of resources but also have a superior defense system, individuals lacking resources or with strong defenses are resistant to becoming infected, while those capable of supporting infection are termed Susceptible. Each person presents a uniquely different environment for the infecting agent. The properties discussed above, disease, infection, infectious and susceptibility or resistant to becoming infected are all critical to understanding epidemics.