Infection can occur when disease-causing organisms known as pathogens enter a susceptible body, such as a person whose resistance is low because of illness. If the conditions are right for the pathogen, it is able to multiply and overwhelm the body's natural defences and infection occurs.
Superficial infection may produce clinical signs such as fever, purulence (pus) and inflammation (warmth, redness and swelling). Deeper infections may produce other clinical signs, for example respiratory infections often produce coughing sputum. However, infection can be present without any visible signs or symptoms of disease. Because of this you should assume that everyone is potentially infectious and treat everyone in the same way by practising infection control procedures.
Also remember that a person may be infectious before they become unwell (ie during the incubation period). With some infections, people can become chronic carriers and remain infectious.