Not wanting to scare you but there are countless numbers of diseases out there. From the common cold to more serious conditions, the fight against disease goes on all the time in our homes, at the office and in medical laboratories. Whilst we all have a natural immunity it seems that nature decides to create new disease all the time as the old ones change and mutate. Help is on hand with paracetamol to ease the symptoms and antibiotics to cure it and treat it.
Sadly, we, humans, are the worst cases for spreading a disease. Whilst you may have some cases where the environment is responsible you have to look at who made that environment in the first place. For example, the reason why typhoid and cholera was such a problem in the Victorian age and the industrial revolution is because of a lack of drains, proper sewerage control and poisons from the factories.
We have significant ways of testing to see if we have a disease before it becomes serious. A great example of this are the Home StI kits Greenwich area authority puts out for people to test their sexual health.