Infectious disease can be spread in the following ways : 

(i) By Air - Sneezing or cough little droplets thrown in the air carry micrrobes which can cause the spread. For example in common cold, pneumonia etc.

(ii) Water - Contaminated water carry disease causing microbes which can cause infection o. Example, gut diseases such as cholera.

(iii) Sexual Contact. Diseases such as syphilis or AIDS are transmitted by sexual contact.

(iv) By vectors  These are intermediaries which carry the infectious agents from the patient to the host (normal person). Example, mosquitoes (Female Anopheles mosquito) can cause spread of malaria.