Respiratory tract infections are categorized as upper or lower infections depending on their respective differences in anatomical and pathological conditions. Upper respiratory tract infection is mainly a viral infection and is a self-limited disease. However, bacterial infections also exist that develop severe complications accompanied by obstruction of the respiratory tract : e.g., acute epiglottitis, peritonsillar abscess, and retropharyngeal abscess. Caution is also called to newly emerging viral infections observed in recent years, such as human metapneumovirus (hMPV-B2). In the case of lower respiratory tract infections, many consist of bacterial infection. Bronchiectasis, COPD and old pulmonary tuberculosis are commonly seen in the clinical setting. Causative bacteria of chronic lower respiratory tract infections are initially Haemophilus influenzae, pneumococcus, etc.; but these are ultimately replaced by Pseudomonas aeruginosa in instances of repeated infections and antimicrobial therapy. Pseudomonas aeruginosa is intractable and becomes resistant to antimicrobial therapy due to biofilm formation and quorum sensing. In recent years, PRSP and community-acquired MRSA are in increasing trends, creating a further need to focus on respiratory tract infections in the future.

Middle East respiratory syndrome (MERS) first emerged in 2002, and in 2015 an outbreak occurred in South Korea, with the result that MERS has now become a worldwide concern. Acquisition of the infection in the community, healthcare associated infection, and movement of patients from country to country are major reasons which explain its worldwide spread.

H5N1 avian influenza in human beings was first identified in 1998 in Hong Kong. Since then, it has spread not only to Southeast Asia but also to other parts of the world. The emergence of H5N1 influenza has increased global awareness of emerging infectious diseases. Currently H7N9 influenza has emerged in China and surrounding countries.

The tuberculosis (TB) notification rate in Japan decreased to 15.4/100,000 in 2014, but is still high compared with 3-5/100,000 in the European countries and the United States. A total of 19,615 TB patients were newly registered in 2014. As Japan is still an intermediate TB-burden country, we must recognize the ever-present possibility that we may encounter a tuberculosis patient in all clinical settings, in which cases it is necessary to diagnose TB patients as early as possible and provide appropriate treatment.

In recent years an increase in nontuberculous mycobacterial (NTM) diseases has been pointed out. Especially remarkable is an increase of Mycobacterium avium complex (MAC) pulmonary disease developing in women without underlying disease after middle age. Although there is no surveillance of NTM diseases at the national level, the estimated prevalence of some study groups has been reported. According to such reports, the rate was 5.7/100,000 in 2007 but has risen year by year since, reaching an estimated 14.7/100,000 in 2014. Aging, a buildup of patients who remain uncured, a surge of interest in the disease, and progress in diagnosis technology are among the suggested causes behind the increase of NTM diseases. MAC accounts for approximately 80%of pathogens causing NTM diseases, followed by M. kansasii (8%) and M. abscessus (3%). It is difficult to treat NTM diseases other than M. kansasii disease.

Pneumonia is diagnosed by specific respiratory symptoms such as cough, fever, sputum and chest pain, confirmed by new lesion on chest radiograph. Patients should initially be treated empirically, based on the likely pathogens for each patient's background ; but when culture results become available, organism-specific therapy may be possible for some patients. Recently, new technology such as mass spectrometry tools and 16S rRNA sequencing are available for classification and identification of bacteria. For all patients with community-acquired pneumonia, pneumococcus is the most common pathogen. For adults, two kinds of pneumococcal vaccine (23-valent pneumococcal polysaccharide vaccine and 13-valent pneumococcal conjugate vaccine) are available in Japan. Monitoring of pneumococcal serotypes causing the disease provides insight into pathogenesis and guidance for vaccine composition.

Human influenza is an acute febrile illness caused by influenza A or B virus. Influenza can be classified into “seasonal influenza,” “human infectious disease with avian influenza” and “pandemic strain influenza.” Seasonal influenza viruses cause annual epidemics that peak during winter in temperate regions. Human avian influenza is caused by a bird flu virus such as A (H5N1) or A (H7N9). Because a species barrier exists between birds and humans, the virus does not easily spread to humans ; however, these viruses can cause severe illness in humans. Mixtures of genes of human and bird virus create a new virus by the process known as “genetic reassortment.” The new A virus subtype is called “pandemic strain influenza virus,” which induces influenza pandemic worldwide, because most people have little to no immune protection for the new virus. Four neuraminidase inhibitors are currently available in Japan for treatment of all influenza types, even the pandemic strain. Recently, the Japanese government established a new legal framework concerning avian and pandemic influenza, including a preparedness plan for influenza pandemic.

Amid rapid progress in globalization in recent decades, the relationship between infectious diseases and humans is also evolving dramatically. Some infectious diseases are recognized as emerging or re-emerging due to global spread and epidemiologic change. Global response mechanisms are necessary. The International Health Regulations (IHR) are a legally binding instrument which requires member states to develop core capacities for rapid detection and response, including surveillance, laboratories, and risk communication. The IHR aim to prevent, protect against, control and provide a public health response to the international spread of diseases in ways that are commensurate with and restricted to public health risks, and which avoid unnecessary interference with international traffic and trade. The World Health Organization (WHO) plays coordinating roles, works with partners to help member states build capacity, and develops global systems of surveillance and response to public health emergency of international concern (PHEIC). Currently, member states have largely failed to fully implement the IHR core capacity requirements, and WHO is not able to lead a full emergency public health response.