What are the three major infectious diseases?

　　There is no such term as "what are the three major infectious diseases." Common infectious diseases mainly include respiratory infections, gastrointestinal infections, blood-borne infections, contact-transmitted infections, and vector-borne infections. Different types of infectious diseases have different transmission routes, so targeted prevention and control measures are necessary. If symptoms suggestive of infection—such as fever, rash, or vomiting—occur, it is recommended to seek medical attention promptly for accurate diagnosis.

　　1. Respiratory infections: Spread through droplets or respiratory secretions, including influenza, tuberculosis, and COVID-19. Symptoms typically include cough, sore throat, and fever. These infections are more common in winter and spring. Wearing masks and maintaining social distancing are effective preventive measures.

　　2. Gastrointestinal infections: Transmitted via contaminated food or water sources, such as norovirus infection, bacterial dysentery, and hepatitis A. Common symptoms include vomiting, diarrhea, and abdominal pain. Key preventive measures include maintaining food hygiene and washing hands before meals and after using the toilet.

　　3. Blood-borne infections: Spread through contact with blood or bodily fluids, such as hepatitis B, hepatitis C, and HIV/AIDS. These often lack obvious early symptoms. Avoiding sharing needles, ensuring safe blood transfusion and donation practices, and practicing personal protection can reduce infection risks.

　　4. Contact-transmitted infections: Spread through direct or indirect contact, such as hand-foot-mouth disease, measles, and mumps. Children are particularly susceptible, with symptoms including rashes and swollen salivary glands. Regular disinfection and avoiding contact with patients' secretions help prevent infection.

　　5. Vector-borne infections: Transmitted through insect bites, such as dengue fever, Japanese encephalitis, and malaria. These are more prevalent during summer and autumn. Symptoms may include fever, headache, and rash. Effective mosquito prevention and control measures can reduce the risk of infection.

　　Daily habits such as frequent handwashing, proper ventilation, balanced diet, regular exercise to boost immunity, timely vaccination according to schedule, avoiding travel to areas with high incidence of infectious diseases, and minimizing contact with suspected cases are essential to build comprehensive protection against infections.