A pathogen (disease causing microbe) enters the body throught the skin, digestive system, respiratory system and reproductive system. Using all the supply they quickly reproduce themselves. This is called the incubation period. If the microbes reproduce, more microbes mean that more toxins are produced resulting in different symptoms. We are said to be diseased.
Cholera - severe diarrhoea + vomiting leading to dehydration
Food poisoning - stomach pains, diarrhoea, vomiting
Influenza - headaches, runny nose, coughing…