The microbial agents move from person to person in a variety of ways. Hence the diseases caused by them are known as communicable diseases. Some of the modes by which they spread are as follows:
Through air:
A number of microbes that cause diseases spread through the air. This occurs via the water droplets thrown out by an individual when sneezes or coughs. This is also known as droplet infection. When a healthy person standing near the person who coughs or sneezes inhale these droplets, these microbes start a new infection. These air-borne diseases are more common in crowded areas, areas that lack proper ventilation and close proximity to the infected person.
Pneumonia, common cold and tuberculosis etc.
Though water:
Diseases can also spread through water. This occurs when the excreta of an infected person get mixed with the drinking water. In diseases like cholera, when the excreta mixes with the drinking water, the microbes cause cholera to enter the new hosts and cause disease in them. These types of diseases spread more likely in the absence of safe supplies of water, and polluted drinking water.
Through physical contact:
Sexual contact and other casual contacts such as handshakes, hugs etc., are some of the physical contacts. As compared to the others, sexual contacts is one of the closest physical contact between two people. Microbial diseases such as AIDS, syphilis and gonorrhoea spread by this mode. These sexually transmitted diseases are not spread by any other physical contacts.
Spread of AIDS:
Apart from sexual contact AIDS which is caused by HIV can also spread through blood-to-blood contact with the infected person. These involve blood transfusions use of an infected needle or razor. It can also pass from an infected mother to her baby during pregnancy or breastfeeding.
Through carriers:
Apart from humans, there are millions of other creatures in the environment. These creatures are sometimes involved in transmitting the infecting agents from one person to another. These animals which act as carriers are known as vectors.
Mosquitoes are the most common vectors. Malaria, caused by the protozoan Plasmodium vivax, is transmitted from one person to another by the female Anopheles mosquito. This mosquito requires highly nutritious food in the form of blood so that it can lay mature eggs. Hence, they feed on warm-blooded animals, including humans and subsequently transfer the disease from the infected person to a healthy individual.

Rabies is caused by the bite of a rabid animal like a dog. Hence, dog is the vector of rabies disease.
The way of spread of rabies through dog bite