What is denialism?Read it all!
Suggesting scientists have some ulterior motive for their research or they are part of some conspiracy. The most basic example of this lie is to say that if the scientists discovered contrary findings they would lose their funding. The most severe example is to suggest scientists are engaged in some kind of elaborate "cover-up" or that they are part of the zionist conspiracy against the Aryan race. Whatever, it amounts to the same thing.
Denialists will often cite: a critical paper supporting their idea, or famously discredited or flawed papers meant to make the field look like a it's based on weak research. Quote mining is also an example of "selective" argument, by using a statement out of context, just like using papers or data out of context, they are able to sow confusion.
3. The fake expert
A bought-scientist or scientist/expert from an unrelated field to say that their data, lack of data, proven-flawed data or their expert opinion disproves the validity of the entire field.
4. Impossible expectations/Moving Goalposts
The use of the absence of complete and absolute knowledge to prevent implementation of sound policies, or acceptance of an idea or a theory. It's a little bit like argument ad ignorantiam, but more sinister. Basically, the suggestion is made that until a subject is understood completely and totally (usually requiring a level of knowledge only found in deities), no action can be reasonably taken.
5. The logical fallacy
The fallacies usually used are metaphor/argument from analogy, appeals to consequence, straw men or red herrings. The metaphor, as hopefully I've demonstrated, is a useful tool in language to help communicate ideas in common sense terms. However, it isn't an argument in and of itself. Denialists will often use argument from metaphor or analogy to suggest that scientific data are wrong. For example, creationists will use as an argument the metaphor that saying natural selection leading to humans is like saying it's probable that you could assemble a jumbo-jet that could fly simply by shaking the constituent parts in a box for 5 billion years. Or that a mousetrap is too complex for evolution because if a single part was missing it wouldn't work.
The whole XMRV/HGRV train-wreck by the way has one important twist on number 4: The absence of complete knowledge is used to demand action against "Human Gamma-Retroviruses" – because they could be there!