Respiratory tract infections (RTIs) are actually a spectrum of diseases associated with infection of both the upper (URI or URTI) and lower respiratory tract (LRI or LRTI) and include the common cold, otitis media, influenza-like illness, croup, bronchiolitis, and pneumonia and are among the most common and important problems in clinical medicine. Lower respiratory infections, such as pneumonia, tend to be far more severe than upper respiratory infections, such as the common cold.

RTIs are caused by a variety of microorganisms including bacteria, viruses and fungi.

Viruses are perhaps the most common causative agents of RTIs, given that 70-80% of most RTIs are viral in origin, with bacterial RTIs as secondary, superimposed infections, which also cause complications. Most viruses can cause infections which present the same symptoms or pathologies.

Bacterial URTIs are usually acute infections which can resolve with little or no treatment.

LRTIs caused by bacteria are usually serious and chronic, especially when treatment is also delayed. Examples are Tuberculosis caused by Mycobacterium tuberculosis and Pertusis (whooping cough) caused by Bordetella pertusis. The well-known pneumonia can be caused by three different bacteria: Steptococcus pneumoniae, Klebsiella pneumoniae and Histoplasma capsulatum. Both infections are common in patients with AIDS or other immunodeficiency states.

Are the evidence linking climatic factors such as temperature, precipitation, and sea-level rise, to the lifecycles of infectious diseases, including both direct and indirect associations via ecological processes? Many studies demonstrate seasonal fluctuations in infectious diseases but few have documented long-term trends in climate-disease associations.

Although international conventions prohibit the use of biological agents for offensive purposes, it is known that many terrorist groups continue their research about the possible use of biological agents as bioweapons. Biological agents have a unique feature when compared to other non-conventional weapons (chemical or radiological); with the exception of toxins, they are able to multiply in the host and in turn be transmitted to other individuals.