The American Heritage Dictionary defines a hypothesis as, "a tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further investigation." This means a hypothesis is the stepping stone to a soon-to-be proven theory. For a hypothesis to be considered a scientific hypothesis, it must be proven through the scientific method. Like anything else in life, there are many paths to take to get to the same ending. Let's take a look at the different types of hypotheses that can be employed when seeking to prove a new theory.
Longitudinal studies in Ghana demonstrate an increase in immunological disorders as it grew more affluent and presumably cleaner.  These results have been replicated by Weinberg et al. who amassed data from a variety of African countries comparing urban and rural environments as well as high and low socioeconomic status (SES). In all four countries urban and high SES groups had a higher prevalence of exercise induced bronchospasm.  The use of antibiotics in the first year of life has been linked to asthma and other allergic diseases.  The use of antibacterial cleaning products has also been associated with higher incidence of asthma . Increased asthma rates are associated with birth by Caesarean section .   The data supporting links to antibiotic use and caesarean section (but not to antibacterial use) are rapidly strengthening.  
Experimental methods are used to demonstrate causal relationships between variables. In an experiment, the researcher systematically manipulates a variable of interest (known as the independent variable) and measures the effect on another variable (known as the dependent variable). Unlike correlational studies, which can only be used to determine if there is a relationship between two variables, experimental methods can be used to determine the actual nature of the relationship. That is to say that if changes in one variable actually cause another to change.