"COVID-19 is a new disease and we are still learning about how it spreads" according to the US Centers for Disease Control and Prevention (CDC) [source]
In general, respiratory virus infection can occur through: [source]
"The virus is thought to spread mainly from person-to-person.
This idea, that large droplets of virus-laden mucus are the primary mode of transmission, guides the US CDC's advice to maintain at least a 6-foot distance: "Maintaining good social distance (about 6 feet) is very important in preventing the spread of COVID-19" [source]
Some experts contacted by LiveScience think that 6 feet (1.8 meters) is not enough [source]
A study of transmission occurring in a restaurant between people at a distance above 1 meter, observed that "strong airflow from the air conditioner could have propagated droplets" [source]
"It is assumed that temperature and humidity modulate the viability of viruses by affecting the properties of viral surface proteins and lipid membrane" [source] Relative humidity (RH, or Saturation Ratio: the state of vapor equilibrium in room air) affects all infectious droplets with respiratory viruses, independent of their source (respiratory tract or aerosolized from any fluid) and location (in air or settled on surfaces). Relative humidity therefore affects all transmission ways but has the most pronounced effect on airborne transmission. [source]
"Measurements of indoor humidities in 40 residential apartments in New York (19) and in 6 high-quality commercial buildings in the Midwest (20) showed indoor vapor pressure of below 10 mb or indoor RH of below 24% in the winter" [source]
Experiments conducted in a study indicated "a striking correlation of the stability of winter viruses at low RH (20–50%), while the stability of summer or all-year viruses enhanced at higher RH (80%)" [source]
The WHO states that "Respiratory infections can be transmitted through droplets of different sizes: when the droplet particles are >5-10 μm in diameter they are referred to as respiratory droplets, and when they are <5μm in diameter, they are referred to as droplet nuclei. According to current evidence, COVID-19 virus is primarily transmitted between people through respiratory droplets and contact routes" [source]
The WHO defines airborne transmission as "the spread of an infectious agent caused by the dissemination of droplet nuclei that remain infectious when suspended in air over long distances and time" [source]
"The maximum transmission distance of SARS-CoV-2 aerosol might be 4 m" (13.1 feet), according to a study published on April 10 on Emerging Infectious Diseases, a journal of the US CDC which also found that "SARS-CoV-2 was widely distributed in the air and on object surfaces in both the ICU and general ward (GW), implying a potentially high infection risk for medical staff and other close contacts" [source]
This is true in a hospital setting and doesn't necessarily apply to other settings. The WHO says that "in the context of COVID-19, airborne transmission may be possible in specific circumstances and settings in which procedures or support treatments that generate aerosols are performed; i.e., endotracheal intubation, bronchoscopy, open suctioning, administration of nebulized treatment, manual ventilation before intubation, turning the patient to the prone position, disconnecting the patient from the ventilator, non-invasive positive-pressure ventilation, tracheostomy, and cardiopulmonary resuscitation" [source]
And concludes that "further studies are needed to determine whether it is possible to detect COVID-19 virus in air samples from patient rooms where no procedures or support treatments that generate aerosols are ongoing" [source]
Virus can remain viable "in aerosols up to 3 hours" found a study published in The New England Journal of Medicine on March 17 [source]
The WHO notes that these findings need to be interpreted carefully: "in this experimental study, aerosols were generated using a three-jet Collison nebulizer and fed into a Goldberg drum under controlled laboratory conditions. This is a high-powered machine that does not reflect normal human cough conditions. Further, the finding of COVID-19 virus in aerosol particles up to 3 hours does not reflect a clinical setting in which aerosol-generating procedures are performed - that is, this was an experimentally induced aerosol-generating procedure" [source]
The virus could spread by touching an object or surface with virus present from an infected person, and then touching the mouth, nose or eyes.
Surface contamination as observed in the study cited above [source]:
76.5% of all personal items sampled at the University of Nebraska Medical Center (UNMC) were determined to be positive for SARS-CoV-2 [source]
Of these samples, 81.3% of the miscellaneous personal items were positive by PCR, which included:
Although the virus titer was greatly reduced, viable SARS-CoV-2 was measured for this length of time:
"The rate of positivity was relatively high for floor swab samples (ICU 7/10, 70%; GW 2/13, 15.4%), perhaps because of gravity and air flow causing most virus droplets to float to the ground.
In addition, as medical staff walk around the ward, the virus can be tracked all over the floor, as indicated by the 100% rate of positivity from the floor in the pharmacy, where there were no patients.
Furthermore, half of the samples from the soles of the ICU medical staff shoes tested positive. Therefore, the soles of medical staff shoes might function as carriers. The 3 weak positive results from the floor of dressing room 4 might also arise from these carriers. We highly recommend that persons disinfect shoe soles before walking out of wards containing COVID-19 patients." [source]