Humans get Lyme disease from infected ticks. Ticks get infected by

Ticks and Lyme Disease

Step 1: Identify the Question

The question states that the information in the passage will support a particular answer choice. That indicates an Inference question.

Step 2: Deconstruct the Argument

ticks feed on animals w/ Lyme, ticks give Lyme to humans

trans. from animal → tick:

- usually rare

- common w/ WF mice

less biodiversity → ticks mostly feed on WF mice

Because this is an Inference problem, there is no conclusion in the argument. Instead, the argument states a series of facts. Lyme is transmitted from animals to ticks to humans. White-footed mice are likely to give Lyme to ticks, while other animals aren’t. Also, ticks feed more on white-footed mice in areas with less biodiversity.

Step 3: Pause and State the Goal

In an Inference problem, the right answer will be something you can prove to be true using only the information in the argument. Identify one answer that can be proven true, and four answers that are not necessarily true.

Step 4: Work from Wrong to Righti

(A) This seems reasonable, because the argument states that humans get Lyme disease from infected ticks. However, is it necessarily true that if there are many infected humans in an area, there must be a high proportion of infected ticks in the area? There are at least two scenarios in which this isn’t true. Perhaps many people in one place contract Lyme disease while on vacation, even though there are no infected ticks where they live. After they return home, there could be many people with Lyme disease in a particular area, but no infected ticks. Also, there could be a relatively small proportion of infected ticks in an area, but humans could be bitten by infected ticks at an unusually high frequency. Perhaps there are just a lot of ticks meaning that even a small proportion that are infected may lead to a lot of Lyme disease in humans. Alternatively, people in these areas may encounter ticks (e.g. go hiking a lot) more frequently than people in other areas.

(B) According to the logic of the argument, if there are no white-footed mice in an area, transmission of Lyme disease from host animals to ticks should be very rare. However, this does not imply that other animals—aside from ticks—don’t have Lyme disease. It just implies that ticks don’t get Lyme disease from them, regardless of whether they have it. The argument doesn’t describe the way that the host animals get Lyme disease in the first place, so they could have contracted Lyme from some other source.

(C) CORRECT. Since ticks only get Lyme disease from other animals extremely rarely, the odds that a tick will carry Lyme disease is directly related to how often it feeds on white-footed mice. In areas with low biodiversity, white-footed mice are relatively more common. Therefore, in areas with more biodiversity, white-footed mice will be relatively less common, and ticks will feed on them less frequently. The ticks in these areas will be less likely to have Lyme disease. In areas with a smaller proportion of Lyme-infected ticks, humans will be at less risk of contracting the disease.

(D) The argument states that when white-footed mouse populations greatly expand, they become the main food source for ticks. However, the argument doesn’t specify what happens when white-footed mouse populations are relatively low. It’s possible that ticks would still regularly feed on white-footed mice, even if other host species are present.

(E) This is untrue, according to the argument. In areas of greater biodiversity, white-footed mice are relatively less common. White-footed mice are much more likely than other animals to transmit Lyme to ticks. With fewer white-footed mice, the average animal will actually be less likely to transmit Lyme to ticks.