Common cold is a viral infection that starts in the upper respiratory tract, sometimes spreads to the lower structures, and may contribute secondary infeeti9ns in the eyes or middle ears. The main differences between the common cold and other respiratory infections are the absence of fever and the relative mildness of the symptoms. About 200 different strains of virus are capable of producing colds. Frequently two or more different viruses can be isolated during a single episode. The cold is spread by person-to-person contact. People can carry the virus and communicate it without themselves experiencing any of the symptoms. Incubation is short—usually one to four days. The viruses start spreading from an infected person before the symptoms appear, and the spread reaches its peak during the symptomatic phase. The incidence of colds peaks during the autumn, and minor epidemics commonly Occur throughout the winter. The reason for this incidence is unknown it may not stem mainly from stresses imposed by chilly weather but rather result from the greater amount of time spent indoors, which increases the likelihood of close contact with those persons carrying cold viruses. Pathologic changes occurring in the mucous membrane that lines the nose, the nasal sinuses, the nasoharynx, and other upper respiratory passages may include tissue swelling, congestion of blood, and oozing of fluids. During the acute phase of the disease, the respiratory secretions are. altered by increase in serum proteins. Parts of cells may 'also be found in the fluids. Tissue repair is rapid and seems complete, although a relationship might exist between colds and more serious respiratory conditions. Cold symptoms vary from person to person, but in the individual the same symptoms tend to recur in succeeding bouts of infection. Manifestations may include sneezing, headaches, fatigue, chilling, sore throat, inflammation of the nose (rhinitis), and nasal discharge. There is usually no fever. The symptoms usually last for only a few days. Tile nasal discharge is the first warning. The secretions become watery, clear, and excessive. Later, they thicken, increase in mucus and pus content, and may colour a yellow-green, with traces of blood. Coughing can be dry or produce amounts of mucus. Other, more serious diseases with similar general symptoms may be mistaken for a cold some of these are tuberculosis, bronchitis, lung abscesses, and inflammation around the heart. Treatment is, in most instances, directed toward allaying of symptoms, coupled with rest and adequate fluid intake. Occasionally antibiotics are given to prevent secondary infections. According to this passage, common cold is ______.