Infection and immunity:

Infection and immunity: The result of any infection depends partly on the virulence and numbers of the invading organisms and partly on the state of the patients defences against them. The state of defences depends upon the two factors: the degree of natural immunity inherited from the parents, which may be reduced by such general influences as malnutrition, worry and overwork and whether there is any acquired immunity resulting from the previous infection with this particular organism. Most infectious diseases confer life long specific immunity. So that second attacks are very unusual. Furthermore, suitable injections of killed or weakened organisms or their toxins may stimulate this specific immunity without actively causing any clinical illness: this fact is the basis of the immunization procedures which are so important in the prevention of diseases such as diptheria, tetanus, poliomyelitis and typhoid. Passive immunization means conferring temporary protection against a diseases by injecting serum which has been taken from someone with an active immunity and which therefore contains a high concentrated of specific antibodies.
Carriers:Some people who are themselves immune from a particular infectious diseases may nevertheless harbour the specific organisms and transmit them from time to time to susceptible persons, either by direct contact or by infecting food or water. This carrier state is particularly important in the spread of typhoid fever.
Routes of infection: The invading organism usually gains access to the patient either in inspired air “droplet infection” or in contaminated food or drink.
Incubation period: After gaining access to the tissues of a susceptible individual, the invading consolidate their position before the symptoms of the diseases appear. This interval is known as the different infectious diseases, but remains in the constant in each of them. The approximate length fever is as follows below:
Less than 7 days Diptheria, Scarlet fever, Measles
10 to 14 days Whooping cough, Small pox, Enteric fever
14 to 21 days Chicken pox, German measles, Mumps.
It is important to know the incubation period of the various specific fevers, since it is occasionally helpful in diagnosis if the date of exposure to infection is known and also because it enables one to forecast the possible appearance of the diseases in people who have been in contact with the small pox it may be advisable to isolate such contacts until it is known whether or not they have contracted the diseases: the duration of this quarantine is worked out by adding a few days to the known incubation period.