What is meant by Infection

Infection is the  invasion and multiplication of microorganisms in body tissues and the reaction to host tissues causing local cellular injury due to competitive metabolism, toxins, intracellular replication, or antigen-antibody response.  Infections are caused by infectious agents including viruses, viroids, prions, bacteria, nematodes such as parasitic roundworms and pinworms, arthropods such as ticks, mites, fleas, and lice, fungi such as ringworm, and other macro parasites such as tapeworms and other helminths. Clinicians classify infectious microorganisms or microbes according to the status of host defenses - either as primary pathogens or as opportunistic pathogens.
Primary pathogens cause disease as a result of their presence or activity within the normal, healthy host, and their intrinsic virulence  to reproduce and spread. Opportunistic pathogens can cause an infectious disease in a host with depressed resistance. Opportunistic infection may be caused by microbes ordinarily in contact with the host, such as pathogenic bacteria or fungi in the gastrointestinal or the upper respiratory tract, and they may also result from (otherwise innocuous) microbes acquired from other hosts (as in Clostridium difficile colitis) or from the environment as a result of traumatic introduction (as in surgical wound infections or compound fractures).

   Chain of Infection 
 Many of the most common primary pathogens of humans only infect humans, however many serious diseases are caused by organisms acquired from the environment or which infect non-human hosts. In general, viral infections are systemic. This means they involve many different parts of the body or more than one body system at the same time; i.e. a runny nose, sinus congestion, cough, body aches etc. They can be local at times as in viral conjunctivitis or "pink eye" and herpes. Only a few viral infections are painful, like herpes. The pain of viral infections is often described as itchy or burning. Bacterial infection are localized redness, heat, swelling and pain. One of the hallmarks of a bacterial infection is local pain, pain that is in a specific part of the body. Bacterial throat pain is often characterized by more pain on one side of the throat. An ear infection is more likely to be diagnosed as bacterial if the pain occurs in only one ear.  Below representation shows stages of infection.        

 
Any sort of infection, small or large should not be neglected as it can cause major hamper to health and body defence system if kept untreated for longer period. People suffering from communicable infections should avoid contact and further spread of contagious pathogen.