United Airlines aircraft are equipped with state-of-the-art circulation systems, similar to those found in hospitals, which use a high-efficiency (HEPA) filter to circulate the air and remove more than 99% of airborne particles. The airline is also adjusting its inflight service to limit person-to-person contamination. This includes handing beverages directly to customers instead of allowing customers to touch a tray and ensuring all flight attendants wear gloves during service.

This is the message United Airlines has been emailing to their passengers:

All aircraft are cleaned at a variety of touchpoints throughout the day.
The cleaning procedure for flights includes a thorough wipe-down of all hard surfaces touched by customers and employees — including lavatories, galleys, tray tables, window shades, and armrests.
United uses an effective, high-grade disinfectant and multi-purpose cleaner.
When we are advised by the CDC of an employee or customer who has traveled on board and who is potentially exhibiting coronavirus symptoms, that aircraft is taken out of service and sent through a full decontamination process that includes our standard cleaning procedures plus washing ceilings and overhead bins and scrubbing the interior
Soon, we will start using an electrostatic fogger to disinfect the air and surfaces within the cabin on all international arrivals into our U.S. Hubs, Honolulu and Guam.
To limit person-to-person contamination, we have instituted the following procedures on board:
Customers may now see flight attendants wearing gloves during food and beverage service as well as during pickup, in all cabins.
We have stopped refilling used cups and glasses in all cabins. If a customer requests a refill, our flight attendants will provide a new cup or glass.
Our flight attendants will hand all beverages directly to the customer, instead of allowing the customer to take their own from the tray.
All tableware, dishes, cutlery, carts, and glassware are washed and sanitized.
We’ve added supplies for our crews on segments flown to Alert Level 2 zones* and upward: gloves, masks, alcohol-based hand sanitizer, Sani-Com wipes, foaming hand soap, and disinfectant wipes as supply become available.
Alert Level 2 zones, as defined by the CDC, can be found here.

In airports

Providing hand sanitizer for use in our crew and break rooms, lounges, and gates
Ensuring regular disinfection of common surfaces inside our airport terminals

What you can do

According to the CDC, the National Safety Council, and the WHO:

Wash your hands often – and thoroughly – with soap and water for at least 20 seconds
Alcohol-based hand sanitizer with at least 60% alcohol content is a good secondary option
Cover your nose and mouth when coughing or sneezing
Avoid touching your nose, mouth and eyes with unwashed hands
Use gloves and masks as needed
Get a flu shot if you haven’t already
If you’re sick – stay home