In human history, there have been several times of influenza raging, which have caused tens of millions of deaths and brought serious social and economic burdens. Although with the development of science, the emergence of vaccines has significantly reduced the incidence and mortality of influenza, due to the high variability of viruses, there is still a lack of effective treatment. More and more studies have found that bacterial pneumonia secondary to influenza was an important cause of the progression to critical illness or even death. Hence, diagnosis and treatment timely of secondary bacterial pneumonia are valuable. Therefore, we discuss the pathogens of bacterial pneumonia secondary to influenza, associated morbidity, mortality, and risk factors. Hopefully, it can provide some valuable references for clinical practice. Since some clinical studies have not separated pneumonia from lower respiratory tract infections, we will discuss these two situations together.

A secondary bacterial infection of the influenza virus in the lungs is a key cause of exacerbation and death. The pathogenesis is characterized by complex interactions between co-infecting pathogens and the host, leading to the destruction of the physical barrier of the airways and the dysregulation of the immune response. This article will review the progress of the mechanism of secondary bacterial infection of influenza virus and provide strategies for preventing the co-infection of influenza virus and bacteria.

H9N2 avian influenza viruses cause worldwide infections in animals including humans and show a threat as a pandemic infection. Since 1998 more than 59 cases including one death due to H9N2 infection had been reported worldwide and the majority of confirmed cases were young children. Due to the large host variety, tolerance to both poultry and mammals and widespread gene reassortment, H9N2 viruses played a crucial role in worldwide infection. In this review, we discuss the current worldwide infection of H9N2 avian influenza viruses as well as their host range, pathogenesis, epidemiology, diagnosis, control, and its pandemic potential.

Infections caused by hepatitis C virus (HCV) lead to persistent infection in most of the times and may cause fibrosis of liver, cirrhosis and hepatocellular carcinoma. At present, there is no vaccine available for prevention or treatment of it. With the development of newer and effective anti viral drugs, most of the infections are curable. But a proportion of infections may progress to cirrhosis of liver /or hepatocelllar carcinoma in spite of being cured from the infection. So, early diagnosis of HCV infections and treatment with effective antiviral drugs is required for prevention of end stage liver disease.

Gastrointestinal tract infection with CMV can occur from mouth to anal canal. In the immunocompetent subjects, the upper GI tract is most commonly involved while immunocompromised individuals have colon as the most common site of involvement. In a study of CMV infection among AIDS patients, the most common site of CMV infection in the GI tract was the colon (55%), while gastric involvement was reported in 40% of cases. CMV mononucleosis, hepatitis and pneumonitis are among the most common manifestations.' GI tract involvement is common and it is easy to access for taking the diagnostic biopsies. Severity and extent of involvement within the GI tract is variable.

Respiratory viruses (RV) have become an important cause of community-acquired pneumonia in immunocompromised patients with the highest rates of morbidity and mortality. The advances in molecular diagnostic methods have increased our understanding of the role of viruses in pneumonia. However, little is known about their impact on patients with human immunodeficiency virus (HIV) infection. In this review, we focus on the most prevalent RV that has been implicated in viral respiratory infections, particularly in pneumonia in HIV infected patients. We discuss the epidemiologic characteristics and clinical presentations of these viral infections and the most appropriate diagnostic approaches and therapies if available.