Diseases are an ever-fresh and intriguing subject. People encounter different diseases on a daily basis, ranging from easy, asymptomatic ones to serious, mortal illnesses. This work deals with contagious diseases most often encountered in children. They can also vary from light diseases with no consequences to serious contagions demanding long treatment. Bacteria, viruses and other causes provide an insight into the genesis of the disease. Starting with the manner of transmission of pathogenic micro-organisms, most often through droplet spread, fecal-oral and direct contact, but also in many other ways, up until the appearance of first symptoms, which are usually general, such as increased bodily temperature, diarrhoea and vomiting. After this, specific symptoms indicating a particular disease and helping the diagnostic process start. The most common children's diseases caused viruses are described in more detail in separate chapters, namely measles, rubella, varicella, rota virus, infectious erythema, polio, mumps and infectious mononucleosis. A separate chapter is also dedicated to other infectious diseases, such as scarlet fever, streptococcus throat infection, bacterial meningitis, pertussis, erysipelas and amoebiasis. Once the disease is diagnosed, treatment can start. The treatment can be conducted with antibiotic and anti-viral drugs (etiologically), which depends on the agent of the disease, or symptomatically, working on discrete symptoms. Many of the agents of infectious diseases are becoming drug-resistant, which represents a huge challenge for medicine. At the very end, the work deals with the inevitable subject of our times, i.e. vaccination in order to prevent different diseases. Vaccination has become controversial, with both supporters and opponents ardently fighting for their attitudes, often disregarding the advice of qualified medicinal experts and the results of scientific research.