What kind of machine learning algorithm can I use?
A couple ideas:


If you have a large data set of tweets, you could try using latent semantic indexing to find out which terms are semantically related based on their usage and co-occurrence. Then after transformation, you can apply some document similarity metric, e.g. cosine similarity, to find tweets most relevant to your queries, e.g. "California measles".
Use knowledge or lexical databases like DBPedia and WordNet to calculate semantic relatedness between your query and the text of your tweets, or to identify tweets with related concepts.