Modes of Transmission of Infection

Generally each infectious disease has a specific route for spreading, for example plague is spread by rodents, and malaria by mosquitoes. There are however, some diseases which can spread by more than one route. For example, AIDS can spread by sexual contact, by blood transfusion, as well as from mother to the foetus in her womb. Infections which can spread by more than one route have that much more chances of survival and transmission. kt us briefly look into the different modes of transmission Of an infectious disease. 

Direct Transmission 

This means that the agent of infection passes directly from the reservoir or source into the body of the host. There are five different ways in which this can happen : 

1. Direct Contact : This means a direct and immediate transfer of the infectious agent from the reservoir or source to another person. This can happen in skin- to-skin contact as by touching, kissing or sexual intercourse. Examples of some diseases that get transmitted by direct contact are : AIDS, leprosy and various skin and eye infections. 
2. Droplet Infections :  This is a direct spraying of droplets of saliva or of the nasal secretions during sneezing, coughing, speaking, spitting or talking, into the surrounding environment. These droplets may fall directly on the mucous membrane of the eyes, mouth, nose or on the skin of someone else close by, and since they  may contain millions of bacteria and viruses, they become a source of infection for others. Examples of infections that spread through droplets are the common cold, diphtheria, whooping cough, tuberculosis, eruptive fevers of all varieties like chickenpox, etc. The chances of such spread of an infection by droplets is increased in conditions of over-crowding where people are close to each other, and in places where ventilation is poor. 
3. Contact with soil : In this case, exposure of susceptible tissues directly to agents of disease or infection present in the soil, compost or decaying vegetable matter, conveys the infection to the host. Some examples are hook worm, tetanus infection and fungal infection. 
4. Inocculation into Skin or the Mucous Membrane : In this case, the agent of disease finds a direct entrance into the skin or mucous membrane. Rabies virus infection by a dog bite is one example, and hepatitis B as well as AIDS virus infections through contaminated needles or syringes are thc others. Kala-azar caused by the bite of the sand fly is also transmitted this way. 
5. Transplacental Transmission : In this case pathogens can get into the placenta from the mother's body. It is a form of direct transmission.  Rubella and herpes viruses, hepatitis B, syphilis and AIDS are examples of infections that can be transmitted to the placenta  from the mother's  body. 

Indirect Transmission 

In this case, the disease agent first gets out through something and then that 'something'  carries it into the new host. Generally,"tiles, fingers, fomites, food and faeces" (the five Fs) are the traditional means for indirect transmission from reservoir to man. This, however, Can occur in many different ways 

1. Vehicle-borne : This means transmission of the infectious agent through something which carries it, like water and ice; food of different kinds particularly raw vegetables, fruits, milk and milk products; blood serum and plasma; biological products like tissues and organs. Of these, water and food are the biggest culprits because of their universal  usage. Diseases spread by  food and water are mostly infections of the gastrointestinal tract such as diarrhoeas, typhoid fever, cholera, hepatitis A, food poisoning and intestinal parasites. Common among those transmitted by blood are hepatitis B, malaria, syphilis and AIDS. Organ transplants may also result in the introduction of disease agents into the receiver of the organ. 
2. Vector-borne : A vector is a living carrier that carries an infectious agent to a susceptible person. This transmission may be mechanical or biological. In mechanical  transmission, the vector only acts as a carrier of the pathogen. For example, the infectious material getting stuck to the feet or proboscis of a fly, or getting into its gastrointestinal tract and passing out with its excreta onto food items. In this case, there is no development or multiplication of the agent on or within the vector. In biological transmission,  the pathogen passes through a developmental cycle or multiplication within the vector. This requires a period of incubation inside the vector before the infectious agent can pass onto another host. Malaria parasite is an example of an agent that gets transmitted biologically through a vector which is a mosquito. We have invertebrate vectors like flies, mosquitoes, fleas, sucking lice, bugs, ticks and mites, and vertebrate vectors like mice and bats. 
3. Air-borne : There are two forms of air-borne infection : one is by way of droplet-nuclei, wherein tiny particles of dried residue of droplets are formed either by evaporation of droplets coughed or sneezed into the air, or by deliberate generation by atomising devices (aerosols). They may also be formed accidentally in microbiological laboratories, in abattoirs or autopsy rooms. The droplet-nuclei may remain air-borne for long periods of time, some retaining and some loosing infectivity or virulence. They keep floating in the air and may reach distant places with the air currents. Some of the diseases spread by droplet-nuclei are tuberculosis, chickenpox, measles, Q-fever, influenza and other respiratory infections. Another form of air-borne infection is dust. During talking, coughing or sneezing, the larger droplets settle down because of their sheer weight on the floor, carpets, furniture, clothes, bedding, and other objects in the immediate environment, and become part of the dust. Many pathogenic bacteria, viruses and spores of fungi as well as squamous cells of the skin are found in the dust of hospital wards and living rooms. Some of the pathogens may survive in the dust for fairly long periods under favorable temperature and moisture conditions. While sweeping, dusting and bed- making the dust becomes air-borne again. Dust particles may also be blown from the soil by wind; this may include fungal spores. Some of the diseases carried by infected dust include streptococcus infections (pneumonia, scarlet fever, rheumatic fever, throat infection) and staphylococcal infections (bacterial food poisoning),  Q-fever (transmitted by ticks) and psittacosis (spreads to man through birds, specially parrots and causes symptoms like influenza). Air-borne dust is primarily inhaled. It may also settle on uncovered food, milk and water, thereby contaminating them; hence the importance of keeping eatables, milk and water, covered. 
4. Fomite-borne : The inanimate articles or substances other than water or food which get infected by infectious discharges from a patient and which harbour these infectious agents and then transfer them to another healthy person, are called  fomites. These include soiled clothes, towels, linen, handkerchiefs, cups, spoons, pencils, books, toys, water glasses, door handles, taps, lavatory chains; syringes, instruments and surgical dressings-just  about every thing that comes in contact with your hands or orifices of the body. Diseases transmitted by fomites include diphtheria, typhoid fever, bacillary dysentery, hepatitis A, eye and skin infections. 
5. Unclean hands and fingers : These are the most common means of spreading infection. The transmission of infectious agents can either take place directly, as in the case of putting infected fingers in mouth or cleaning a wound with an infected finger, or indirectly by contaminating food, water, vessels or linen with your infected hands. Some of the diseases that very often spread this way are typhoid fever, dysentery, hepatitis A and intestinal parasites. Whole epidemics can be caused by unclean hands and fingers!

Mode of transmission