• It is a disease which is capable of being transmitted directly or indirectly from one person to another.
• Directly means Spread of disease By having Contact with a person
• Indirectly means spread of disease by having contact with an inanimate object
If a number of organisms and its virulence is greater than the defence power of the body. The organism can multiply and produce a disease.
• Such a person in which a defence power is lacking is called a susceptible person
• A person in which a defence power is sufficient enough to destroy the organism is called the immune person.
The time interval between the entry of an infectious agent into the host and the appearance of first symptoms of the disease. The duration varies from case to case depending upon the virulence of the agent and also on resistance or susceptibility of the host.
• It is the time interval during which an infectious agent can be transferred from reservoir (patient) to the susceptible host.
• It is a French word and refers to the inanimate objects such as bedding, pillows, towels, books, spoon, forks etc.
It is one which is capable of transmitting the disease under natural condition .e.g. specious of Anopheles mosquito.
• Carrier is a person who harbours the microorganism of disease and excretes them without himself suffering from symptoms.
Types of Carriers:
Carriers are of two type
1: convalescent carrier
A person who suffered from communicable disease but who in their convalesce still excrete the organism e.g. a patient who had typhoid but during convalescence he excretes salmonella typhi in his faeces.
Such carriers are temporary for few weeks but only some become permanent
2: Symptomless carrier
Those people who never had his illness or symptom but harbours and excrete the organism. In such cases infection produces subclinical attack with no symptom e.g. vibrio cholera or poliovirus .it can be temporary or permanent.
Other types are:
There are different types of carriers.
e.g diphtheria streptococcus, staphylococcus
The spread of Infection:
A model is used to understand the infectious process.
The links of this model are
• The infectious agent
• A reservoir (a carrier or inanimate)
• Portal of exit from the reservoir
• Mode of transmission of infection
• Portal of entry
• Susceptible host
Sources of infection:
Exit of the organism
The common routes are
· The organism of nose and throat escape through this route
· In intestinal diseases like polio dysentery cholera and typhoid the organism escape through stool
· In urinary tract disease
· The infecting organism is present in peripheral blood so in insect also tack it through sucking
The entrance of the organism to the susceptible host:
The organism enters the susceptible host through
· 1: inhalation
§ Organism is present in the nose, throat and mouth of the patient so after coughing, sneezing or even talking loudly these are disseminated to a susceptible person it is also called droplet infection e.g. influenza, common cold, pulmonary T.B and whooping cough.
· 2: ingestion
§ Water, milk and food infected with organism carry person to person by taking or drinking such infected foods.
§ The person get a disease e.g. enteric fever cholera, dysentery and polio.
§ guinea worm infection is caused by drinking infected milk.
· 3: inoculation
§ By skin biting the insects suckles blood of the infected individual and get a disease like malaria, sandfly fever, dengue, kalazar , yellow fever and sleeping sickness and transfer them to others
Transmission of infection
· Three conditions are necessary for the spread of the infection
§ The organism responsible must be able to leave the body of the infected host
§ The organism must be transferred by some agency to the susceptible person. In the majority, some vehicle is required like air, water, or food or contaminated clothes or other objects
§ The organism must gain entry into the body of the host.
§ There are four ways of entry into the new host.
§ 1. Through placenta
§ 2. Through Respiratory tract
§ 3. Through intestinal tract
§ 4. Through surface epithelium i.e. skin