Lyme disease, or Lyme Borreliosis, is caused by bacteria from the genus Borrelia. The type of most interest to those residing in the US is Borrelia burgdorferi sensu stricto, more commonly shortened to Borrelia burgdorferi. The disease is transmitted by ticks of the Ixodes species which carry the bacteria in their midgut. Tick feeding, as occurs when they are hooked onto our skin, encourages bacterial replication, subsequent migration to the tick salivary glands, and finally bacterial transmission from tick to humans (or other animals) - this process can take several hours. If the tick is removed in a timely manner, generally the first two hours, there is a low probability of transmission; however, prolonged attachment increases the risk.
The site of the tick bite typically becomes marked by a bulls-eye rash, where the bite marks the center of the bulls-eye. Other relatively common symptoms include flu-like symptoms and are generally non-specific. If left untreated, or if spread occurs, a number of other conditions may occur; however, this is not common.
The bacteria does not produce a toxin but rather seems to exert damage by causing a strong (host) inflammatory response -- the intensity of which varies across different Borrelia genospecies. It is also interesting to note that some individuals may have a genetic predisposition to some complications such as Lyme arthritis and possibly other conditions.
The bacteria is unique in that it has special virulence factors which facilitate evasion of our immune system by "its ability to downregulate expression of specific immunogenic surface-exposed proteins, including OspC, and to alter rapidly and continually by recombination of the antigenic properties of a surface lipoprotein known as variable major protein-like sequence expressed (VlsE)."
What remains peculiar, however, is that some patients experience symptoms for prolonged periods, while others may experience problems after an initial asymptomatic period -- depending on the situation this is referred to as post-treatment symptoms or post-Lyme Borreliosis syndrome. Interestingly, there has even been some association between infection and cutaneous B-cell lymphomas.
In a recent review on the topic published in the Lancet, the authors discuss the term chronic Lyme disease -- a term they describe as poorly defined yet widely used. It is indeed true that because of instances of delayed Lyme disease clinical manifestations, as well as the post-Lyme Borreliosis syndrome, some may feel compelled to use the term chronic Lyme disease. However, as the authors point out, the ambiguous use of the term has led to significant misinformation about the disease on the Internet, leading to an increasing number of patients with "medically unexplained symptoms" being diagnosed with chronic Lyme disease (even in the absence of relevant medical history or evidence).
Though we have a substantial understanding about Lyme disease and its cause, a number of peculiarities remain, such as the persistence of long-term symptoms in some individuals. The difficulty in properly identifying patients and correctly diagnosing them is further compounded by the often nonspecific nature of these symptoms, including "persistence of fatigue, musculoskeletal pain, or difficulties with concentration and memory".
Fortunately, Lyme disease is relatively benign and only infrequently results in prolonged symptoms. Even then, most patients experience only mild symptoms such as fatigue. Treatment is via antibiotics, most notably doxycycline; a tetracycline antibiotic. The best way to avoid having to think about Lyme disease is by trying to remain covered when hiking in the woods to avoid tick bites, particularly in the northeastern United States, and properly removing any ticks as soon as possible.